Hello! Please, help me with this 🙏 Is it correct to use the emphatic construction this way?
'It was these subjects I really liked'
'It was those people who gave me a lift'
'It was my students who found me'

It's just the fact that the word following 'it was' is in plural that confuses me and makes me think that it is supposed to be changed into 'Those were' or something like this.

Question

Hello! Please, help me with this 🙏 Is it correct to use the emphatic construction this way? 
'It was these subjects I really liked'
'It was those people who gave me a lift'
'It was my students who found me'

It's just the fact that the word following 'it was' is in plural that confuses me and makes me think that it is supposed to be changed into 'Those were' or something like this.

Answer

"It" is functioning as an empty subject in all three of your examples, and "it" is always singular. The words after "was" in all three are serving as predicate nominatives. The verb always needs to agree with the subject, not the predicate nominative.

Answer

It’s correct. It is strange though now you mention it. “Those” is plural

Answer

All three sentences are correct. Consider these sentences:

What was it that you really liked? ("What" is singular but doesn't require that the answer be singular)
I really liked these subjects.  (no problem here. you can think of "these subjects" as a plural or as the single thing that is liked, a collection (singular noun) of subjects)
I really liked these. (same)
It was these that I liked. (same sentence, really)
It was these subjects that I liked. ("subjects" just adds a bit of clarification)

Answer

Thanks for your prompt responce! You've helped me a lot. Now I can sleep well :)

Answer

Looks rite to me. Sounds rigid when spoken out loud but you got it.