If you speak German, some English grammar rules will feel natural and others will feel confusing. These are the biggest differences to focus on first.
German puts the verb at the end of subordinate clauses — 'Ich weiß, dass er gestern kam' means 'I know that he yesterday came'. English always puts the verb before time expressions. German speakers must restructure these clauses mentally.
German has definite articles for three genders (der, die, das) that also change depending on grammatical case. English has just 'the', 'a', and 'an' with no gender or case changes.
German separable verbs split apart in a sentence — 'Ich rufe dich an' means 'I call you up' but the 'an' floats to the end. English phrasal verbs stay together. German speakers find English verb patterns surprisingly simple in comparison.