When learning a new language it is very important to us that our teachers tell us when we make mistakes, using the wrong tense or the incorrect prepositions.
Please "fix" my grammar if I make any mistakes!
Can you "rectify" my grammar, please?
I need help improving my grammar!
外国語を学ぶとき、ミス（例えば時制や前置詞の間違いなど）をしたときに先生に指摘してもらうことは非常に重要です。
Please "fix" my grammar if I make any mistakes!（文法が間違っていたら直してください）
Can you "rectify" my grammar, please?（文法を直していただけますか）
I need help improving my grammar!（文法を上達させたいです）
"If I mistake may you correct my grammar?"
'May you' is a polite way of phrasing 'can you' ex. 'May you help me with the cleaning today' 'May you take me to the shops'
"If I mistake may you correct my grammar?"（文法を間違えたら、直してもらえますか）
'May you' は 'Can you'（してもらえますか）の丁寧な言い方です。
'May you help me with the cleaning today?'（今日掃除を手伝ってもらえますか）
'May you take me to the shops?'（買い物に連れて行ってくれますか）
You can simply ask your teacher this way as it is polite and by using 'any' indicates all of your mistakes.
"Please correct any grammar mistakes I make."
上記のように先生にお願いできます。シンプルで丁寧な言い方です。
'any' は「あらゆる間違い」のニュアンスです。
"Please correct any grammar mistakes I make."（文法を間違えたらその都度指摘してください）
Please correct my grammar every time i make a mistake
Every time i make a mistake please correct me
When learning a new lesson mistakes will happen so by asking someone to 'correct' you , you are making sure you are getting it right
every time suggest you want every mistake even if little corrected
using please is a nice polite way of asking
Please correct my grammar every time i make a mistake
Please correct any grammar mistakes I make.
訳：私のどんな文法の間違いも修正してください。
any にはどんな～にもという意味がありますので、細かいものも含めてすべて直してほしいというニュアンスが伝わりやすいです。
Please correct my grammar every time I make a mistake.
訳：私が間違ったら毎回文法を直してください。
こちらは every time I make a mistake わたしが間違ったときはいつも という意味ですので、間違った度に正してほしいという感じですね。
少しニュアンスが異なりますが、いずれも文法を直してほしいということが伝わります。
少しでも参考になれば光栄です。
Could you let me know every time I make a mistake, please?
Please tell me immediately when I make any kind of error in either pronunciation or grammar
During an online English lesson, you want to ask your teacher to correct your grammar every time I make a mistake. There are a couple of suggestions above which may be appropriate.
Both of these are clear and polite ways of telling your teacher to correct your grammar every time you make a mistake. By using either "all" or "any" it makes it clear that you want every mistake corrected.
Could you correct me everytime I make a mistake, please?
If I mistake may you correct my grammar, please?
In the case like yours, I would advise you to use any of written examples by me. Both of them will help the teacher to understand what you want from them.
日本 
南アフリカ 
イギリス 
アイルランド 
アメリカ合衆国 
