a few mistakes in a sentence I've come across

[Englishlanguage]

I've come across this sentence on the web:

Studies show that memorizing grammar rules don’t help you to learn the practical use of a language.

Shouldn't it be doesn't instead of don't?
And shouldn't it be usage instead of use?
Thank you

[Englishlanguage]

So the word usage refers to the grammar of the language, while use refers to the ability of speaking it, am I right?

[hdrao]

‘The usage of American spellings in British publications is to be deprecated,’ ‘The usage of water for washing cars is forbidden.’ Incorrect. The word required is use.

Usage means ‘that which has become customary’ or ‘that which has been established by long- standing use. Thus, though ‘it is me’ violates the grammatical rule that the verb to be should be followed by a complement in the nominative case, it has become established by usage, and must therefore be accepted.

There cannot be a wrong usage of a word, but there may be a wrong use of it.