I'd say sentence structure is the most important part to learn in English grammar. Understanding how to form sentences—using subjects, verbs, objects, and correct word order—forms the foundation for clear communication. Along with that, mastering tenses and articles (a, an, the) is essential for expressing time and specificity accurately. Once these areas are solid, you can build on more advanced grammar concepts.

It really depends on your needs and English level, but I'd recommend that the tenses (past, present, future) are the basics that you should start with. A professional teacher can develop a lesson plan for you to help. Please feel free to contact me if you'd like to start learning and improving your skills: I am a professional English teacher from the UK. And if you use this promo code for a trial lesson with me, you will receive a $5 discount: B6cAHE