Context: My book says that:
The copular verb is absent in the present tense for a compound predicate and the noun [which is the complement] is in the nominative case. In the past and future tenses there must be a copular verb быть (был, буду), стать (стал, стану).
Note that the above says nothing about which case the noun should be in for the past or future tenses. The book, however, gives the following examples of correct usage.
Мой брат -- журналист.
Мой брат был журналистом.
Мой брат будет журналистом.
Question 1: Does this mean that the following examples are incorrect?
Мой брат -- журналистом.
Мой брат был журналист.
Мой брат будет журналист.
I am pretty sure that the first sentence is incorrect, but I am not sure about the last two, because (1) I could have sworn I have seen sentences written with nominative in the past and future tenses, and (2) as mentioned above the book doesn't explicitly exclude the possibility of them being correct (in contrast to the first sentence, when it says that the noun must be in the nominative).
Question 2: When in general is the instrumental used with complements or copular verbs? Only in the past or future? Must it be used in those cases, or is it also correct to use nominative?
I am especially confused because I did not know that the instrumental could be used with быть or стать in any tense -- I thought it was only used for this meaning with the verb называться in very formal writing. /Question 2
Note: I translated the passage from my book from German, so it may not be entirely correct. In particular, it uses the grammatical term "Prädikatsnomen" which I am not sure how to translate, although this is mostly because the meaning of the word "predicate" in English is ambiguous.