Boit is used when referring to he/she/it. Bois is used when saying I/you. So 'I drink' or 'You drink' would be Je bois / Tu bois. The boy drinks / she drinks would be "Le garcon boit / Elle boit". Keep in mind that I'm referring to the singular/informal 'you' here, there's a difference when it's a plural / formal you in french. Hope this helps!
In the English sentence "The boy is drinking milk.", "is drinking" is the present progressive form of "to drink".
In French, there is no continuous tense (for example, we say: "Je mange", whether in a definite or indefinite period of time.).
You can use the idiomatic phrase "être en train de" to express the idea of continuity. (ex: "Je suis en train de manger." for "I am eating.")
If you hover over the French tense (ex: "(je) mange"), you will get the hints for the English conjugated tense ("am eating").
So, if the meaning of the sentence is "in general", then use the English present simple, for ex: "(In general) I eat rice."
If the meaning of the sentence is "in a definite moment", then use the English present continuous, for ex: "(Now) the boy is drinking milk."
FYI, in French, there are two forms that are related to the English gerund:
- "participe présent": the "participe présent" of "manger" is "mangeant".
e.g.: "Mangeant des fruits, il est en bonne santé" = "(By) eating fruits, he is healthy".
- "gérondif": the "gérondif" of "manger" is "en mangeant".
e.g.: "Il parle en mangeant" = "He speaks while eating".
In this case, "du" is a partitive article, and is a contraction of "de + le" (literally "of the").
It can be translated into:
"Le garçon boit du lait." :
"The boy is drinking milk." (No article in English)
"The boy is drinking some milk."
"The boy is drinking some of the milk." (as: "part of the")
It differs from the definite article "le":
"Le garçon boit le lait." = "The boy is drinking the milk."
They are conjugations of the verb Boire=To Drink It is in the 3rd group of verbs where the infinitive ends in -RE. It conjugates thus: Je Bois (I drink/am drinking), Tu Bois (You,familiar form, drink/are drinking), Il/Elle Boit (He/She drinks/is drinking), Nous Buvons (We drink/are drinking), Vous Buvez (You, formal form, drink/are drinking), Ils/Elles Boivent (They drink/are drinking) There are three main groups of verbs: Those infinitives ending in -ER, those ending in -IR and those ending in -RE (there are more to this group... a lesson in itself really) Two useful sites: "www.about.com/fr grammar verb conjugator" and "French Lesson 2: Conjugation of -ER verbs, subject pronouns by Rafatheman" (This gets you to his site through this lesson and you can then scroll for any of the other lessons on this site, all of which are very palatable and wholly compliment this, Duo's course)