I found the following sentence:> È un vero peccato che ti abbiano rubato il portafoglio.> Ora non possiamo comprare da mangiare.However, I wonder why this uses `comprare da mangiare`, instead of `comprare a mangiare`. I know both `a` and `da` can be used as a proposition before an infinitive verb. I also know that there is a usage of `da + infinitive` to express "something to do" much like `to + infinitive` in English.However, then I wonder why `da mangiare` can work without an object. For example, if the sentence is the following, which has an object to modify, I have no problem in understanding it:> Ora non possiamo comprare qualcosa da mangiare.However, if there is no object there, I can't differentiate when to use `a` and when to use `da`, especially since both can be used to express an aim after the main verb.So how can I tell apart the use cases of these two prepositions? (Please answer the question in English if possible...)
The two different way to say it, and they must be both considered as names, not as verbs even if they are built with an infinitive verb attached to them. They are called "locutions" (a group of words that semantically work together for an unique meaning)
"qualcosa da mangiare" it's used with a more specific meaning: not everything, but just something, "qualcosa". E.g. "ho una macchina da scrivere" (I have a typing machine); "non ho niente da scrivere" (I have nothing to write about)
"da mangiare" it's more generic, and it works as a plural, like a collective name e.g. "non ho da scrivere" (I have nothing to use for writing)
