If you are a not native, the english grammar may seem difficult to you.
For example verbe tense can produce confusions, hence, you need to remember that there exists three main tenses (Past Tense, Present Tense and Future Tense), each of them having four forms. Past tense has the following forms: Simple Past, Past Continuous,Past Perfect,Past Perfect Continuous. Present Tense has the following forms: Simple Present, Present Continuous, Present Perfect, Present Perfect Continuous. Future Tense has the following forms: Simple Future, Future Continuous, Future Perfect, Future Perfect Continuous.
Subject-verb agreement can also produce mistakes if the rule singular subject - singular verb is not followed.
The misplacement of modifiers (adjectives and adverbs) can also become a problem in the composition of a sentence.