Avoir and être don't use their infinitive as the futur stem (that would give "avoirai" and "êtrerai", neither exists). Instead they have their own short, irregular stems, but the endings are the same regular ones you already know from our futur simple lesson.
1. Avoir: stem aur-
| J'aurai 30 ans l'an prochain. | I'll be 30 next year. |
| Tu auras le temps ? | Will you have time? |
| Nous aurons une réponse bientôt. | We'll have an answer soon. |
2. Être: stem ser-
| Je serai en retard. | I'll be late. |
| Elle sera médecin. | She'll be a doctor. |
| Vous serez les bienvenus. | You'll be welcome. |
3. A common expression: il y aura
Il y a (there is/are) becomes il y aura (there will be) in the futur, the same aur- stem as avoir.
| Il y aura beaucoup de monde. | There will be a lot of people. |
4. Conjugation: avoir & être
| Pronoun | Avoir | Être |
|---|
| je / j' | aurai | serai |
| tu | auras | seras |
| il / elle / on | aura | sera |
| nous | aurons | serons |
| vous | aurez | serez |
| ils / elles | auront | seront |