Is there a rule of thumbs for verbs followed by “à” or “de”?

Question

Is there a way in which you can work out which verbs are followed by de and which verbs are followed by à without having to learn and memorise them all?

For example: parler de and aider à

Is there an easy way to remember which prepositions follow the verbs?

Answer 1

In your example, those are just two different meanings. For example "Nous avons parlé de foot à mon fils" means "We talked about soccer to my son". As you can see they have different functions and can be used in combination. Nothing to memorize here, you just need to understand the function of the preposition.

For the verb "aider" I can not find an example using both that is commonly said. "J'ai reçu l'aide de mon père à porter mes valises", is not usual (maybe even wrong). "J'ai reçu l'aide de mon père pour porter mes valises" or "Mon père m'a aidé à porter mes valises" are both correct. The first translates to "I got help from my father to carry my luggages", while the second translates to "My father helped me to carry my luggages".

You can also use it for directions like "il y a moins de deux heures de route de Cannes à Marseille."

Answer 2

When it comes to prepositions (or grammatical cases in inflected languages) there are no rigid rules - their use is idiosyncratic to each specific language.

As a general guideline one could use the overlap of functions of prepositions in the target language with those in the language one already knows, e.g., à can be often translated as to, whereas de can be translated as of/about, so we have mappings like:

talk to somebody about something -> parler à quelqu'un de quelque chose

However, one can readily come with examples where this rule breaks, like

try to do something -> essayer de faire quelque chose
think of somebody-> pense à quelqu'un

And then, of course, there are such completely diverging uses as:

tarte aux pommes -> apple pie (pie with apples)

One can however to learn about specific grammatical situations, mandating use of one or another preposition:

• the preposition mandated by verbs: one simply have to memorize which verb uses which preposition, like dire à quelqu'un, parler à quelqu'un, essayer de faire quelque chose, etc.
• genitive structure / possession: in French it is mostly built using de: le fils de professeur, le maire de la ville, etc.
• direction of movement: aller à Paris = go to Paris, venir de Paris = come from Paris
• other specific uses like pain au chocolat/raisin
• and so on.