Hi Mohamad! Spanish, like English, has some collocations. As in English you have to say "I have to" or "You are good at" and not "You are good in", in Spanish you have to use some of them as well. Sometimes a verb has more depending prepositions than just one with different meanings. So, you can't mix them up. The best way to learn them is just using them little by little. "Tener que" is similar to "have to". "Olvidarse de hacer algo". With "a" you can say: Salir a correr. Quedar a comer. Volver a fumar. Empezar a nadar. Parar a comprar algo. All these verbs have something to do with movement or change. I hope this brief explanation was useful for you :)

This is something you just have to get to grips as you study the language, there are no shortcuts or easy rules that I'm aware of. As you study the language more, you'll get a feel for which verbs can go with which prepositions. The best way is to consider the verb and the preposition together. So for example to say 'to have to do something' in Spanish the construction is 'tener que hacer algo.' That's just how it is. No, 'a' is used with many verbs. For example, empezar a, comenzar a, aprender a, enseñar a... No, they are not interchangeable. In your example you've used 'dejar.' Now, 'dejar' has more than one meaning. 'Dejáme saber' means 'let me know.' But 'dejar' can also mean 'to stop.' So 'deja de hacer eso' means 'stop doing that.'