Learn English – Which sentence is wrong

Standing on the train platform today, waiting for Nick, you might say, anxiously:

“If Nick is here we will not miss the train.”

If you narrate this tomorrow, you will backshift the tenses:

“Yesterday I said that if Nick was here we would not miss the train.”

Now suppose that in fact Nick is not on time. Now your condition, your IF clause, is an irrealis or counterfactual. Just before the train leaves you might say, despairingly:

“If Nick were here we would not miss the train.”

And afterwards, as the train pulls out of the station, leaving you on the platform, you might say bitterly:

“If Nick had been here we would not have missed the train.”

What is confusing here is that would does double duty: it may act as either the past indicative form or the present conditional form of will. You have to figure out which is in play from the context.

I could not find anything better than this on Dictionary.com

It says that 'for' in such context has been used as a conjunction meaning 'because, since'. It's old usage of the word 'for'.

[Middle English, from Old English; see per in Indo-European roots.]

Usage Note: 'For' has been used as a conjunction meaning "because, since" for over 1,000 years. It is familiar in many famous quotations, from the New Testament's beatitudes (Blessed are the meek: for they shall inherit the earth, Matthew 5:05) to Shakespeare's sonnets (For thy sweet love rememb'red such wealth brings / That then I scorn to change my state with kings).

Today this use of for is rare in speech and informal writing, and it often lends a literary tone or note of formality. · Like the word so, for can be viewed as either a subordinating or a coordinating conjunction, and it has been treated variously as such. It has the meaning of a subordinating conjunction, since it clearly subordinates the clause that follows it to the previous clause or sentence. But like a coordinating conjunction, for has a fixed position in the sentence, and its clause cannot be transposed to precede the superordinate clause containing the main idea. It is ungrammatical in present-day English to say For they shall inherit the earth: blessed are the meek. Perhaps because of this ambiguity in function, for is treated variously with regard to punctuation. Sometimes it begins a dependent clause and follows a comma, and sometimes it begins an independent clause (as if it were a conjunctive adverb like moreover) and follows a semicolon or period (when it is capitalized as the first word of a new sentence). All treatments are acceptable in standard usage. The difference is really one of emphasis: starting a new sentence with for tends to call more attention to the thought that it introduces.

So,

I didn't go to school today for I felt ill.

is not weird because you want to say that you did not go to school today because/since you felt ill.