The infinitive in Spanish is tricky because in English it's used in a different way... I think that the most effective way to learn it is to learn verb phrases (in Spanish, perífrasis o frases verbales). For example: After tener que, deber, poder, querer, ir a, empezar a, terminar de and many others, you should always use the infinitive. Some examples: Tengo que estudiar. Debes llamar a tu padre. Terminamos de comer a las ocho. But there are some verbs that require a gerund, such as continuar, seguir, andar, estar. English speakers usually say "continuar a hacer", while the correct phrase would be "continuar haciendo". Some examples of this would be: Seguí estudiando durante dos años más. ¿Qué andas haciendo estos días? There are more complicated cases. If you post some of your mistakes maybe I can be more specific, but in the meantime I hope this helps!