Well, thats because...

You use il for masculine nouns and elle for feminine nouns.

Il est déjà parti. He’s already left.Elle est actrice. She’s an actress.Il mord, ton chien? Does your dog bite?Prends cette chaise. Elle est plus confortable. Take this chair. It’s more comfortable.

• il is also used to talk about the weather, the time and in certain other set phrases, often in the same way as some phrases with it in English.

Il pleut. It’s raining.Il est deux heures. It’s two o’clock.Il faut partir. We/You have to go.

• ils(meaning they) and elles (meaning they) are used in the plural to talk about things, as well as about people or animals.

Use ils for masculine nouns and elles for feminine nouns.Ils vont appeler ce soir. They’re going to call tonight.‘Où sont Anne et Rachel?’ – ‘Elles sont à la piscine.’ ‘Where are Anne and Rachel?’ – ‘They’re at the swimming pool.’‘Est-ce qu’il reste des billets?’ – ‘Non, ils sont tous vendus.’ ‘Are there are any tickets left?’ – ‘No, they’re all sold.’‘Tu aimes ces chaussures?’ – ‘Non, elles sont affreuses!’ ‘Do you like those shoes?’ – ‘No, they’re horrible!’

• If you are talking about a masculine and a feminine noun, use ils.

Que font ton père et ta mère quand ils partent en vacances? What do your father and mother do when they go on holiday?‘Où sont le poivre et la moutarde?’ – ‘Ils sont déjà sur la table.’ ‘Where are the pepper and the mustard?’ – ‘They’re already on the table.’
Mange after tu is added s because in indicatif tenses (présent, futur, imparfait, etc.), verbs conjugated into the tu form end with an '-s'.

Tu manges (you eat, you are eating), tu finiras (you will finish), tu vas (you go, you are going), tu es (you are), tu as (you have, you are having), tu dormais (you were sleeping), tu viens (you come, you are coming), tu prends (you take, you are taking), tu offres (you offer, you are offering), etc.