how do you know when to use mange or manges? please let me know.
It depends on the "person" conjugating the verb. In English, you have different conjugations for I/You/We and He/She/It, right? Well, for French (and a lot of other languages) there's a different conjugation for each single person. In the case of manger, in the Present, it's: Je mange Tu manges Il/Elle/On mange Nous mangeons Vous mangez Ils/Elles mangent
There are rules though, so it's not hard to learn the conjugations. Good luck!
Worth getting to grips with this - using the link from Vu7xENbj as a similar pattern of word endings is relevant for many verbs - and the principle (though not all the endings!) applies to all verbs.
When you're refering to a "him", "her", or "it", you will use mange.
Example: "Sparky is eating." would be "Sparky mange."
When you're refering to the person you are talking to, you will use manges.
Example: "You are eating." would be "Tu manges."