1.Conocer and conocer + a (Conocer and the personal a) Conocer is irregular in the “yo” form. In Spanish, when a person is the direct object of a verb, the preposition a comes before it. ¿Por qué no invitó a Paula?
2.¿Conoces tú a alguien famoso?
3.Present progressive (Present progressive) to describe what “is happening”, estar + present participle (in English present participles end in -ing, and in Spanish they end in -ando or -iendo) If the stem ends in a vowel, the vowel changes to what? a “y” What are the present participles of dormir and servir? ¿Quiénes están abriendo la piñata? ¿Dónde están celebrando la fiesta de Celia?

2. Sí, yo conozco a alguien famoso.

With 3, I’m unsure what you are exactly asking

With 4, there is some context I am missing

Explanation:

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well, if he says unos it is 1, if he says dos, it is 2, if he says tres, it is 3, if he says cuatro, it is 4