If you speak Korean, some English grammar rules will feel natural and others will feel confusing. These are the biggest differences to focus on first.
Korean is SOV — every sentence ends with the verb. 'Naneun yeong-eoreul baewo' literally means 'I English study'. In English this must become 'I study English'. Every Korean sentence needs to be restructured.
Korean has no articles. It uses special counting words called classifiers instead. English articles 'a', 'an', and 'the' express definiteness — Korean expresses this through context and sentence structure.
Korean verbs carry social information about the relationship between the speaker and listener — levels of respect are built into the verb endings. English verbs carry no social information at all.