conditional constructions - Using the past tense to describe unreal, hypothetical, or impossible situations

I know that when the situation is unreal, we use the "if" clause plus past tense.

If you didn't know English, you would not know how to solve these problems.

[The fact is that the person knows English and probably knows how to do the problem]

My question is that: Will it be ok if I also use past tense to describe an unreal situation without using "if".

He is pretending he didn't know how to speak English.

or

He acts like he didn't know how to speak English.
[the fact is that he knows English]

Answer

{He is pretending/he acts like} he didn't know English.

You may very well encounter past forms like this in speech, where realis and irrealis forms have to be juggled and chosen on the fly. But Standard Written registers call for a realis form here:

{He is pretending/he acts like} he doesn't know English.

Although the pretense of ignorance is known to the speaker to be false, it is presented by the pretender as true, and that is how the speaker represents it. The irrealis sense resides in the main-clause verb, pretend or act like.

In your conditional example, on the other hand, the speaker presents the condition as irrealis, hypothetical or counterfactual, so the condition is expressed with an irrealis form

If you didn't know English you couldn't solve these problems.

Subordinate clauses may fall outside the "scope" of an irrealis expression:

If I were you I would tell him that you won't go unless he gives you extra time off. ... not 'wouldn't go' and 'gave'.