The subject-verb agreement rule requires a noun to match the plurality of the verb that goes with it. Even native English speakers get wrong time to time, especially if the subject and the verb are not right next to each other.
Idiomatic phrases are learned through experience and exposure to the English language, so it’s difficult for non-native speakers of English to know these phrases right off the bat.
Another part of grammar that gives non-native speakers trouble is countable and uncountable nouns. English would be straightforward if every noun could be pluralized by simply adding an -s or -es at the end, but unfortunately, English is not that simple.
There are three tenses: past, present, and future. But it gets a bit complicated with tenses like past perfect, present perfect infinitives, future perfect, and more.
The importance of capitalization is often overlooked by non-native speakers. These are the main places capitalization errors happen: titles, proper nouns and common nouns, the beginning of a sentences and comma splices