If you speak Greek, some English grammar rules will feel natural and others will feel confusing. These are the biggest differences to focus on first.
Greek uses SVO like English but with more flexibility due to its case system. In Greek you can often rearrange words for emphasis. English word order is strict — position determines grammatical role.
Greek has a complex article system with three genders and four cases, making Greek articles one of the most complex in European languages. English articles are much simpler — no gender, no case changes. Greek speakers often find English articles easier than their own.
Greek has two aspect forms — perfective and imperfective — for most verb tenses. English uses different tenses to make these distinctions. Greek speakers must learn to use English tense choices to express the aspect meaning they convey through verb form in Greek.