How do you know when to use "de" and "a" in place of the English "to"? For example, in the sentence "Ellos son difíciles de reconocer". Why use "de" instead of "a", and how would changing "de" to "a" change the meaning of the sentence to you?

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You can't use 'a' in that sentence because we use 'a' when talking about destination or purpose (I think).

For example:
Voy camino a casa. (I'm (on my) way home)
Voy al hotel. (I go/I'm going to the hotel)
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Voy a bañarme. (I'm going to take a shower/bath)
Voy a jugar. (I'm going to play)
Te voy a matar! >:( (I'm going to kill you! >:( -obviously not seriously, just expressing you're very mad at someone 😅😆)

'De' on the other hand has a lot of meanings. You can use 'de' instead of from, for example: Vengo del trabajo. (I come from work). Here 'de' kinda takes the opposite meaning of 'a', becauste it's talking about origin. 'Vengo de españoles' I come from Spanish people. would be correct too.

You use 'de' also, with a verb, when you're not talking about doing that thing now, but talking about the fact of doing that thing. For example. 'Son difíciles de reconocer' (They're hard to recognize) notice how you're not saying YOU are going, or are recognizing them right now, instead, you're saying THE FACT of recognizing them IS hard. So: son dificiles de reconocer. Even though I would prefer saying that like this: 'Es difícil reconocerlos' call me picky but I feel like it sounds a bit more natural. 😅 (The other sentence would be: I am going to recognize them. Los voy A reconocer)

I'm just going to write more sentences with 'de' and their translations bc I'm not that good at explaining. Heheh.

De donde viene? (Where is it coming from?)
De que quieres el helado? (What (flavor) do you want the ice cream?)

OH! You also use 'de' to express possession. Instead of the 's.

Paty's dog. (El perro de Paty)
My mum's pc. (La pc de mi mamá)

So yeah.

I would recommend when learning a language try to learn words like those like new words. Like if they were new in english and you'd have to start to use them (... y tuvieras que empezar A usarlas) in your daily life. Like, not thinking in what's the translation, but thinking how natives use them and try to use them. Because words in different languages don't always have a direct translation into our native. And when they have sometimes it's confusing.

So yeah, I hope that was kind of helpful 😅😭 and

Buena suerte! ;) Trabaja duro! ♥♥♥🌸