Choosing between de and à
I know there is another thread on this... as a matter of fact I think I dimly recall seeing one before... sadly, I can't find it, so I have to make a new one. Recently, prepositions have really been causing great difficulty for me in terms of grammar... in particular, knowing which to use out of these two prepositions. I simply have no idea which one I should use when, or what the difference is. Here are some random sentences, where one or the other is used... but I couldn't say which.
Il est difficile de/à comprendre cette vidéo française (comme moi toujours quand j'etais debutant, et quelquefois maintenant :p).
Elle pense de/à son enfance.
Nous parlons de/à Marc, qui aide très souvent les apprenants de français et anglais sur Duolingo.
Êtes-vous sûr d'essayer de/à chercher des fantômes ?
Thanks in advance if anyone has ideas or useful help
Constructions involving an adjective directly followed by a verb always use de - e.g. Il est difficile de faire.../Il est obligatoire de faire.../Il est nécessaire de faire... etc. In fact, you correctly chose de in one of your later example sentences: Êtes-vous sûr d'essayer de/à chercher des fantômes ?
As for penser and parler - their meaning changes based on which one you choose. The difference in meanings is pretty intuitive for parler:
parler de qqch/qqn - to talk about sth/sb
parler à qqch/qqn - to talk to sth/sb
In other words, the à in parler à exists simply as a conduit for the indirect object.
It's a little weird for penser, since it almost seems flip-flopped:
penser de qqch/qqn - to think about/of, as in, to have an opinion about (→ Qu'est-ce que tu penses de ce film-là ? "What did you think about that movie?")
penser à qqch/qqn - to think about/of, as in, to keep in one's mind (→ Tu penses à cette fille ? "Are you thinking about that girl?")
This difference in meaning you'll just have to memorize for penser.
The last one is trickiest - essayer de vs. essayer à. The answer is essayer de, but there's no particular reason for this other than "just because". Certain verbs just inherently need de, others inherently need à, some need neither, and unfortunately you'll just have to memorize them. So, a couple links for you: