Why is it that in French, sometimes, there is 'a' instead of 'de'?

Question

How do you know which one to use?

i.e. un magnetophone a cassette

un poste de radio

what's the difference???!!!!

Answer 1

Not very easy to explain but semantically, "à" and "de" mean the opposite. The first implies something you aim at, tend to whereas  "de" indicates origin.

So it's logical that the expression "magnétophone à cassette" uses preposition "à" because it is a recorder in which you are going to put a cassette/tape. The recorder comes first, then you add something. In "un poste de radio", radio technology comes first and then the set is a support to operate it. But the origin is the radio.

Other examples:

- du fard à paupière (eye make-up): the paupière (eye-lid) is what you aim at with the fard (powder), it's a goal, not an origin.

- Une boule de cristal : crystal is what the ball is made of. It is the origin, not a goal/aim.

Now I have never heard "un magnétophone à cassette", we usually just say "un magnétophone". Well, not that they're used anymore.