Me ayudas a corregir las frases en español, por favor :)

I can't eat the apple.
No puedo comer la manzana.

Si todos son correctos, por favor dígame. Muchas gracias!

Me ayudó ayudas a corregir las frases en español, por favor :)

The apple is red.
 La manzana es roja. OK   

It is John's apple.
 Es la manzana de John. OK   

I give John the apple. 
    Doy la manzana a John.  Le doy la manzana a John. (As you can see it's the same structure you used         in the fourth sentence, but in this case the corresponding to the 1st person singular instead of 1st person plural)     

We give him the apple. 
    Le damos la manzana. OK

He gives it to John.
    She gives it to him.
    Lo da a John. (Él/Ella) se lo da a John. (Here you are  practicing  with Direct Object(DO) and Indirect Object (IO) Pronouns, " it " is "DO" and John is "IO". In many occasions in Spanish, both the IO Pronoun and the actual IO (John) appear in the same sentence. "Le", which would be the corresponding pronoun is replaced by "Se", so the sentence is written like this: "Le Se lo da a John/a  él.")

Is the apple red? 
    ¿ Es la manzana roja? OK

The apples are red. 
    Las manzanas son rojas. OK

I must give it to him.
    Debo darlo a él. Debo dárselo a él. (Here you have the same situation as in # 5. Here you have used a structure called verbal periphrasis (deber + infinitive),The way you have built the sentence both DO(lo) and IO (se) pronouns have to go together behind the infinitive and in the following order ( OI + OD): dar+se(OI)+lo(OD))  Another way to express the same idea would be : "Se lo debo dar a él."

I want to give it to her. 
    Quiero darlo a ella.(Is there any alternative way to say "darlo a ella"?) Quiero dárselo a ella. (Similar to # 8) or "Se lo quiero dar a ella."

I'm going to leave tomorrow. 
 Voy a salir mañana. (This is a tricky one for various reasons: 1st. The verb to leave in Spanish has the meaning  of "to go away" while "salir" is meant as "getting out of a place". 2nd. The most usual translation of the verb to leave to Spanish is: Irse; and when translating the English structure: to be + going to + infinitive you get: Ir a ... ) So the right way of building this phrase would be "Me voy mañana." 

I can't eat the apple. 
    No puedo comer la manzana. OK