I'm sorry, but in questions of language, and especially of language change, "why" questions don't usually have answers, other than "because that's what happened".
The Early Modern English forms you give had already changed - in Old English, the pattern was very like modern German, but the distinct plural forms had mostly dropped out of use before Shakespeare's time (you still find ye goeth in the King James Bible).
The remaining distinctions are -est and -eth vs -s. The first of these disappeared when thou disappeared. The second was a dialect difference: IIRC -s was Northern, and -eth was Southern, though it may have been more complicated than that.
Edited to remove a wrong claim.
TLDR: The if part of present conditionals never takes a subjunctive form in present-day English, only an indicative one. You’re incorrectly trying to apply Portuguese rules for conditionals in English, but English does not work like Portuguese!
The examples you have presented are not grammatical in present day English because if clauses like the ones you’ve shown take a tensed verb:
The project she presented was a mess. If she *fail with another one she'll be fired.
That is ungrammatical. It needs to be like this:
The project she presented was a mess. If she fails with another one she'll be fired.
In the English of a far more ancient vintage than you will ever casually come across, we did at one point use a present subjunctive inflection there, and later an untensed infinitive, but that was long ago and for the most part far away.
Lusophone Excursions: For speakers of Portuguese only
- WARNING: To illustrate why this is wrong in a way the original poster can best understand it, I below show contrasting examples of the same sort of conditional in English and Portuguese. I also demonstrate that English has changed in its treatment of these over the best few hundred year, but that Portuguese has not. Lastly, I show that Spanish practice has split off from following the Portuguese practice to following the English one during that same period.
You seem to be attempting to “calque” Portuguese conditionals into English. Portuguese employs for these a special form which the Portuguese call the futuro do conjuctivo (future conjunctive) and the Brazilians futuro do subjuntivo (future subjunctive). These are not indicative forms in Portuguese the way they are in English and in Spanish:
- EN: If she fails at another project, she’ll be fired. —present indicative
- PT: Se ela falhar em outro projeto, (ela) será demitida. —future subjunctive
- ES: Si (ella) falla en otro proyecto, será despedida. —present indicative
The Portuguese would obviously* read ela falha not ela falhar there if it were in the indicative not the subjunctive, but this is not supposed to be indicative in Portuguese the way it is supposed to be in English. You’re trying to do the same thing in English, calquing the Portuguese conditional forms into English in ways that don’t make sense to native speakers of English. You should use the indicative in English here, just as speakers of your sister language Spanish now also do.
* “Obvious” to thee and to me, but probably to few other readers here.
Of Boys and Bulls: What happens when your raging bull gravely injures your neighbor
Once upon a time when the language was young, English actually did use a “modally marked form” (call it subjunctive if you must) there in its if clauses, but no longer. By way of example, please consider the following Early Modern English from the King James Version (KJV) of the Bible published in 1611. There we read for verse 29 from Exodus 21:
- [KJV] If an ox gore a man or a woman, that they die: then the ox shall be surely stoned, and his flesh shall not be eaten; but the owner of the ox shall be quit.
That’s what you’re trying to do, but we don’t do that any longer in English. Here’s how that same verse runs in the so-called “King James Version 2000” (KJV2000) translation:
- [KJV2000] If an ox gores a man or a woman, that they die: then the ox shall be surely stoned, and its flesh shall not be eaten; but the owner of the ox shall be clear.
So instead of “if an ox gore” in the Early Modern English, we now say “if an ox gores”. If you try to use the English of 400 years ago, particularly in ordinary conversation, most people will be confused and few will understand you correctly. They’ll think you’re making a mistake — which you would be. It would be just like speaking Portuguese today the way it was spoken back when Camõens wrote Os Lusíadas in the Sixteenth Century. You’d just confuse people.
For cultural comparison, here’s how that verse was translated into the Portuguese of its day by Antônio Pereira de Figueiredo a couple hundred years ago:
- [Pereira] Se hum touro ferir com as suas pontas hum homem, ou huma mulher, e elles morrerem isso, apedrejar-se-ha o touro, e não se lhe comerá a carne; mas o dono o touro será innocente.
Even though that uses the same inflections of its verbs as Modern Portuguese uses, it sure is funny to read and funny to say, isn’t it? The style of speaking and the words chosen have changed considerably as you will see when you compare what you have just now read above with two versions in Modern Portuguese (where the touro has become a boi :) as follows:
[O Livro] Se um boi escornear um homem ou uma mulher, tirandolhe a vida, o boi terá de ser apedrejado e não comerá a sua carne. Mas o dono do animal não será culpado de nada.
[NVI-PT] Se um boi chifrar um homem ou uma mulher, causando-lhe a morte, o boi terá que ser apedrejado até a morte, e a sua carne não poderá ser comida. Mas o dono do boi será absolvido.
So unlike English, Portuguese still uses a subjunctive there just as it did hundreds of years ago. But English does not. The English loss of the subjunctive there parallels the loss of the subjunctive in Spanish over the same period. Here’s how it ran in the Reina-Valera Antigua translation back when Spanish still used the future subjunctive for this sort of if clauses:
- [RVA] Si un buey acorneare hombre ó mujer, y de resultas muriere, el buey será apedreado, y no se comerá su carne; mas el dueño del buey será absuelto.
Even though that old sort of Spanish should be especially easy for Portuguese speakers to read because of its old words and old verb forms, people today no more talk that way in Spanish than they do in English. The Modern Spanish translation in the Castilian NIV runs:
- [CST] Si un toro cornea y mata a un hombre o a una mujer, se matará al toro a pedradas y no se comerá su carne. En tal caso, no se hará responsable al dueño del toro.
Summary
If as a Portuguese speaker you are going to speak English (or even Spanish) in a way that makes sense to native speakers of those other languages, you must shed your Lusophonic sensibilities about how present conditionals work and use only indicative forms not subjunctive ones in the if parts of these sorts of conditionals.
Postscript on calques and calquing
When you calque something from one language to another, you translate its original so literally that it risks losing its sense in the new language. Wiktionary says that a calque is:
A word or phrase in a language formed by word-for-word or morpheme-by-morpheme translation of a word in another language.
The Portuguese Wikipedia entry for that word starts out:
Em linguística, e mais precisamente em lexicologia, etimologia e linguística comparada, chama-se calque, calco ou decalque a um procedimento de formação de palavras que consiste em cunhar novos termos mediante a tradução de vocábulos estrangeiros e conforme as estruturas da língua de origem. É um tipo de empréstimo léxico particular, no qual o termo emprestado foi traduzido literalmente de uma língua para a outra, considerando mais a forma do que a ideia.
Best Answer
As this is a very broad question whose full answer merits several written books, I first present a brief orientation and outline of how we got here today, with pointers to more detailed material.
Closely related to this question are questions like these, some of which you may have actually been asking about indirectly:
In those questions as well as in the one which I believe was asked, synchronic analysis fails to provide a satisfying answer, or really any at all in most cases. Instead one must examine the language diachronically to draw out a sensible answer, and a full treatment of that answer must reach back over six millennia.
Languages with mainly unbound morphemes — atomic units of meaning at the lexemic level only — are classified as analytic languages, while those whose individual words each comprise multiple bound morphemes are classified as synthetic languages.
An individual word in a synthetic language combines several bound morphemes where each little internal piece adds something to the word’s overall meaning. Morphological inflection can occur via affixes (give > giving, ox > oxen, walk > walked), via sound changes of vowels or consonants (give > gave, shoot > shot, man > men, mouse > mice, this > these), or via both (brother > brethren, swell > swollen, give > gavest).
Today’s English is for the most part an analytic language. For meaning, we rely far more strongly on fixed word-order and on little “function” words (including auxiliary verbs, articles and other determiners, conjunctions, and prepositions) than we do on synthesis via inflectional morphology the way synthetic languages do.
At the same time, English still has a few inflections left in it thanks to its ultimate derivation from a long genetic line of highly synthetic languages stretching back over 6,000 years. We can trace English’s ancestors all the way to the prehistoric (read: unwritten) Proto-Indo-European (PIE) language of our distant ancestors. That language was strongly synthetic in all its open word classes, including in its verb forms.
One distinctive PIE verb inflection that occurred in certain verbs’ third-person present singular conjugations was *‑t or *‑ti. This became in prehistoric Proto-Germanic *‑di or *‑þi, in Old English ‑(e)þ, in Middle English ‑(e)þ, and in Early Modern English the ‑(e)th of he liveth, which passed quickly enough into the distinctive inflection you’ve asked about, the ‑(e)s form of he lives or he itches in today’s English.
PIE third-person singular inflections also produced forms like German er bleibt, Latin manet or cōnstat, Old French il remaint, and although you can no longer normally hear it pronounced in speech, also in the imperfect il restait of today’s French (but no longer in its present tense sauf in certain relics such as subjunctive qu’il soit for “that he/it should be”).
Old English was a much more synthetic language than Middle English was, which saw dramatic reductions in inflections as the language transitioned to an analytic one. There are several proposed explanations for why this happened, but that’s a whole nother topic with its own lines of investigation. Suffice it to say that Middle English was a furious time of mergers and acquisitions that saw as sweeping changes to the grammar as to the lexicon.
Similarly to how Middle English remade synthetic Old English analytically by reducing inflections across the board, Early Modern English verbs did enjoy more conjugations than today’s English does, but these again underwent rapid evolution. From the Wikipedia article on that topic we read:
I reckon that that’s as detailed an answer to a rather broad question as one dare get here.
I in passing note that English does retain a single, unique inflectional distinction in the past indicative’s singular was versus its plural were (which is also the past subjunctive irrespective of number). Verbs other than be are no longer so marked.
Postamble
Lastly, in John Lawler’s comment:
When John writes {‑Z₁} using an archiphonemic {Z}, what he means is that that morphological inflection ends up being pronounced in three slightly different ways depending on its surrounding phonologic environment:
Moreover, we use that same {Z} archiphoneme for three of English’s eight remaining inflections:
Second we use it for the plural inflections of singular nouns.
And third we use it for possessives formed via enclitic.
All three of these follow the same pronunciation rules to translate archiphonemic {Z} into actual phonetics. (Please don’t worry about the spelling; spelling is merely an immaterial side-effect of writing technology, and so shouldn’t be paid any attention to here since we’re talking about language not technology.)