Formal:
* Répondre à: This is the most common way to say "to respond to". It is used in both written and spoken French.
* Je réponds à votre question. (I am responding to your question.)
* Il a répondu à l'invitation. (He responded to the invitation.)
* Réagir à: This means "to react to" and can be used to express a more emotional or spontaneous response.
* J'ai réagi à son commentaire. (I reacted to his comment.)
Informal:
* Répondre: You can simply use "répondre" without "à" in informal contexts.
* J'ai répondu. (I responded.)
* Réagir: Like in the formal setting, you can use "réagir" to express a more emotional response.
* J'ai réagi tout de suite. (I reacted right away.)
Other ways to express a response:
* Donner une réponse: This means "to give a response."
* Faire un commentaire: This means "to make a comment."
* Dire quelque chose: This means "to say something."
To express agreement or disagreement:
* Oui, je suis d'accord. (Yes, I agree.)
* Non, je ne suis pas d'accord. (No, I disagree.)
To express understanding:
* Je comprends. (I understand.)
* Je vois. (I see.)
To ask for clarification:
* Pouvez-vous répéter, s'il vous plaît? (Can you repeat, please?)
* Je n'ai pas compris. (I didn't understand.)
Remember to choose the appropriate phrase based on the context and the level of formality.