For the compound noun front + end it is front end:
Noun
front end (plural front ends)
- (computing) that part of a hardware or software system that is closest to the user.
frontend and front-end are alternative forms.
The compound noun front + end + engineering may be another matter.
I already dealt with <GH> pronunciation variation here; <CH> is a more interesting situation because it involves borrowings from familiar European languages, rather than languages written in other orthographies.
The grapheme <C> goes all the way back to the Semitic glyph gimel, the third letter of the original alphabet: 'aleph 'cow', beth 'house', gimel 'camel', etc. 'Aleph represented a glottal stop, a phoneme the Greeks didn't need, so they threw it away and invented vowel letters (which Semitic writing didn't need so much and didn't use).
So they made alpha a vowel letter. Beth became beta and /b/ is /b/, pretty much the same thing. Gimel /g/ became gamma /g/, and the letter still had the same camel-like hump.
When the Romans borrowed Greek letters, alpha became <A>, beta <B>, and gamma <C>. But it no longer meant /g/; it got devoiced to /k/; <C> always represents /k/, in Classical Latin (Medieval Latin is quite another matter).
And that's the last time that <C> always represents anything. When Latin split into the Romance languages, and Latin writing became a standard for other languages, <C> split into many varieties, depending on the original histories of the various languages, what sound changes had occurred when to which one, and which words had been borrowed into which languages (before or after the sound changes). Some of these variations acquired new spellings as <CH>, because the grapheme <H> is often used to differentiate letters.
From the standpoint of English, the various pronunciations of <CH> include:
- [x] (a voiceless velar fricative, which does not exist in Modern English;
but which does occur in German Loch, Scots loch, Hebrew /ləxayim/, and Russian /xoroʃo/)
- /k/, a voiceless velar stop, often a subsitute for [x], as when Americans say Loch Ness /laknɛs/;
but also in words borrowed from Italian, if <CH> is followed by <I> or <E>,
and also in words borrowed from Latin, no matter what follows <CH>.
- /ʃ/, a voiceless alveolopalatal sibilant, in words borrowed from French.
- /tʃ/, a voiceless alveolopalatal affricate, in most native English words;
and also in words borrowed from Spanish.
(This is probably the most common pronunciation)
Best Answer
I originally closevoted with a comment saying the general trend is to move from two separate words, through the hyphenated form to a single-word form. But actually it's a bit more complex than that. Compare this NGram for what I would call a "compound noun" usage at nighttime...
...with the more obviously "compound adjective" usage a nighttime [some modified noun]...
What you see at first glance is that the "noun" usage has declined overall (we've increasingly tended to discard the word time in constructions like "Dracula visited her at night[time]"). On the other hand, we've become increasingly fond of the compound adjectival form.
But if you look more closely, you'll notice that although the single-word form has become the most common in both contexts, there's been a significant shift in relative preferences for the other two forms. As a "noun", the double-word form is now slightly preferred over the hyphenated one, but as an "adjective" the opposite preference is now quite marked.
In light of that I think I should qualify my original comment. The general tendency is indeed to discard the hyphen - but whereas "compound adjective" usages invariably replace it with a single-word form, in other contexts we're quite likely to revert to a two-word form.
This same distinction can be seen if we compare adjectival fleabitten (where the hyphenated form continues to dominate) with the noun fleabite (where the two-word form is now preferred).
I think what this means is that writers in general increasingly reject "indiscriminate" use of hyphenated forms (just as we no longer indulge in the indiscriminate capitalisation of C19 and earlier). But hyphenated compound adjectives are more resistant to this shift than other usages. Because we don't like two-word compound adjectival forms at all, we keep the hyphen unless the single-word form is both familiar (to the ear) and easy to parse (for the eye).