Common Grammar Mistakes in English, Grammatical Errors Confusing the Infinitive, Gerund or Base Form of the Verb Examples; I must to buy a new cartoon book. I must buy a new cartoon book. You use the infinitive with modals like should, must when you should use the base form of the verb but You can also use gerunds when you should use infinitives: Examples; I decided going to the cinema with my mother. I decided to go to the cinema with my mother. Wrong Word Usage This happens mostly because in your native language, the verb can come at the