Articles:
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I often don’t know when to use “a” or “the.” ¶
2.

I can see how this would be difficult for an English learner.
"I am learning a language" refers to a specific language, but I did not name which language. Works fine as a statement introducing a topic.
"I am learning the language" doesn't make sense by itself. But if someone said "What do you think of French?" You could reply with "I am learning the language."
"A girl gave it to me" - some girl but I don't know who she was.
"The girl gave it to me" - maybe I'm in a situation where there are multiple people who could have given it to me, and I'm clarifying it was the person who is a girl.

Singular and Plural Forms:
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The rules for singular and plural forms are not fixed.

"Sheep" and "moose"? 😁

Tenses:
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It is very difficult to determine which tense to use in a specific context.

I have had this difficulty in every language I study.

Synonyms:
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Many words have similar meanings.

It is difficult to choose the most suitable one.

Native speakers have trouble with this, too, so I wouldn't worry too much about this until you get more advanced.

Vocabulary :¶

English has too many words to learn.

So do other languages!