"willst" isn't a real word. Don't use it.
"wilt" is an archaic word, used in second person singular. E.g. "I will", "Thou wilt", "He willeth".
"hath means has and hast means have" isn't quite right.
Again, they are archaic singular conjugations: "I have", "Though hast", "He hath".
I sometimes use would after would which of course is incorrect in terms of grammar.
I believe there's no such grammar rule in English that you should not use would after would. If indeed there's such a rule, you should not blindly accept it, because there are an infinite number of exceptions to the rule as made abundantly clear by previous answers.
*If I ...
This is a very confused question. Let's take it one thing at a time:
Is it possible to use modal verbs in Past Perfect?
This question has no answer because
There is no Past Perfect Tense in English.
Modal verbs are not inflected for tenses anyway.
So, on the face of it, the answer to that question, asked that way, is No.
However, that doesn't appear ...
Even though we call it "the past participle", on its own it has nothing to do with the past. Participles, gerunds and infinitives are all non-finite forms. They carry no tense at all. They easily combine with any tense.
was described -- past
is described -- present
will be described -- future
The "is described" example employs the present ...
It depends on what you want to emphasize.
The snake moved THROUGH the grass.
This sentence emphasizes the motion of the snake.
The snake moved IN the grass.
This sentence emphasizes the location of the snake.
Why use one or the other? It depends on the context of the sentence. If one didn't know HOW the snake moved, one would likely use the first, ...