When is "a" needed before an infinitive?

I have had a frustratingly bothersome question for many months that I have not been able to find an answer to, nor has it been explained in the lessons, so I will ask here and hope that one of our resident experts can enlighten me. I have noticed many sentences in the lessons include an "a" (to) immediately before an infinitive, and probably just as many that do not include the "a," even though the english word "to" is obviously implied. For example, this afternoon I was reviewing lesson 2.8 in the Premium Course and came across the following two sentences: Espero terminar pronto....I hope TO finish soon. No vamos a terminar pronto....we are not going TO finish soon. I can see no grammatical difference between the structure of these sentences (other than one is a positive statement and the other negative), yet the "a" is omitted in the first and included in the second. Please, someone, cause these scales to fall from my eyes! Gracias, Dan

Hola Dan, The "a" is used before an infinitive with a verb of movement or progression ir, volver, venir, etc You can't say voy ir al cine. It's voy a ir al cine.Etc. There other cases where "a" is needed and I by no means could tell you all of them. There is a book titled "¡Búscalo!" that goes into this and a myriad number of other things regarding usage and grammar etc. It can be purchased from Amazon and I highly recommend it as it contains a lot of info. Hope this helps. Saludos, Ricardo

*gruñir*...I knew it was going to be another one of those por o para kind of things that you just have to learn. Well, if learning a language was easy the whole world would be polyglots. Thanks for the explanation, Ricardo. It will help me to know what trigger words to look for now. And also thanks for the book suggestion. I will check it out. Dan

Since I am just memorizing a lot of the sentences, I noticed that after the verb "tener", it is always followed by the word (preposition?) que. It would be nice if this observation is a rule for the verb tener. And yet there is no que in "Tengo hambre"

Hola Aurora, Tener que followed by an infinitive means to "have to" or "must", but tener is used many ways and is not always followed by que. Tengo un lápiz. Tengo hambre y tengo que comer. Saludos, Ricardo