Que/de/a use in verb+infinitive combos

[msquaredb]

I am using Duolingo and becoming frustrated with the lack of explanation on the site and the way it teaches by punishment. So...I am seeking some help here.

My question is regarding the use of que/de/a to precede an infinitive after a verb. Below are some examples that have confused me. To me....they don't seem substantially different, but they use different linkers. I keep getting these types of things wrong in Duolingo and it is frustrating when the rest of the sentence is correct.

Me tengo que despertar a las seis.
I have to wake up at six.

Usted empezó a hablar.
They begin to speak.

Es imposible de saber.
It is impossible to know.

Thanks in advance!

[AngelicaDeAlquezar]

I think this is a struggle in every language. Each verb needs a different "companion" to make sense. I'm afraid you'll have to get used to their specific constructions, which you will learn by identifying different situations and practice.

If it's any consolation, we have trouble when we try to figure out "try to", "dream of", "stay at"... So it's better to let these expressions flow settle down in the brain and over practice time they will come up spontaneously.

[wrholt]

I'm also a native speaker of US English, although I started learning Spanish more than 40 years ago. And I concur with AdA: each verb has its own specific and arbitrary requirements regarding whether or not it permits another verb as a complement (that is, as a completion of its meaning), whether or not it requires another word to connect that complement, and what specific word is required.

Some textbooks try to offer guidelines for predicting which pattern is generally the correct one based on the type of meaning of the verb: for example, verbs that indicate the start of an action typically require using the preposition 'a' to introduce an infinitive as a complement: empezar a, comenzar a.

Unfortunately, these guidelines are not completely reliable. For the most part we must learn the correct patterns for each verb (and sometimes for each specific meaning of a verb) on a case-by-case basis, just as we must learn the gender of many nouns on a case-by-case basis.