Would these two sentences be grammatically correct?

1. How do you know if a sentence is grammatically correct?
2. Can we use would two times in a sentence?
3. What are these grammatically correct?
4. Do this or does these?

How do you know if a sentence is grammatically correct?

In order for a sentence to be grammatically correct, the subject and verb must both be singular or plural. In other words, the subject and verb must agree with one another in their tense. If the subject is in plural form, the verb should also be in plur al form (and vice versa).

Can we use would two times in a sentence?

Yes, it is acceptable to use the word "would" multiple times in a sentence, but given your examples, there are reasons against using it such as to avoid ambiguity. The subjunctive mood allows a speaker to concisely "express various states of unreality," particularly in dependent or subordinate clauses of sentences.

What are these grammatically correct?

This, that, these and those are demonstratives. We use this, that, these and those to point to people and things. This and that are singular. These and those are plural.

Do this or does these?

The short answer is that this is used with singular or uncountable nouns (e.g. this egg), while these refers to plural nouns (e.g. these cookies). This article will explain the difference between this vs these and help you remember how to use these words correctly in your writing.