If you speak Japanese, some English grammar rules will feel natural and others will feel confusing. These are the biggest differences to focus on first.
Japanese is SOV — the verb always comes last. 'Watashi wa eigo wo benkyou shimasu' literally means 'I English study' in English word order. Every English sentence must be restructured from the Japanese pattern.
Japanese has no articles. It uses particles instead — small words that show the grammatical role of each word in a sentence. English articles show whether something is specific (the) or general (a/an). These are completely different systems.
Japanese verbs do not change for person or number — just for tense and politeness level. English verbs change for person and tense but have no built-in politeness forms. Japanese speakers often sound either too formal or too casual in English.