In Japanese, there is a lack of definite articles, a pluralization of nouns and conjugation of plural verbs as well as no separate future tense makes this sentence vague; it could become "the cat drinks the milk," "cats drink milk," or even "the cats will drink milk." So what are we to do?
In the case of nouns, we have a system of numbering related items by type. For instance, cats are small animals, so they have a distinct counting word which can also be used for all small animals. That is to say, things like cats and dogs would have a different counting word than cars or boats. This system is not used as often as one might think; it's only used when the number of a noun needs to be specific.
Occasionally the suffixes like -たち will be used. たち (tachi) means "group" and again, is generally used when a specific group needs to be defined. 猫たち would mean "the group of cats," although using just 猫 would be sufficient to imply cats as a whole. Saying 私たち (watashitachi) would be an acceptable use as it sets apart ME (私) and my group (たち), or simply "we."
Verb conjugation in English is difficult because the verb has to agree with number and tense: I drink, I drank, I will drink, I do not drink, I did not drink, I will not drink, You drink.... etc. Japanese's basic verb conjugations are Present/Future Positive, Present/Future Negative, Past Positive and Past Negative: 飲みます, 飲みません, 飲みました, 飲みませんでした. These conjugations also account for I, you (singular and plural), he/she/it, we, and they.
Now, to reflect the parts of speech, we use post-positional particles. In other words, the particle after the word tells us what its doing in the sentence. は shows us the subject or topic of the sentence. Interestingly, a Japanese sentence does not require a subject; so long as the topic has been introduced and hasn't been changed, we can omit it: 猫はミルクを飲みます。一杯を飲みました。太っています。The cat drinks milk. (It) drank a lot. (It) became (and remains) fat; or even (It) is fat. Our direct object particle を, in this case shows us our direct object (what is affected by a verb) and usually goes before a transitive verb (a verb that needs a subject and an object to work).
Hopefully this was enlightening, and I'm going to be editing and adding to this and all posts constantly.
猫 - neko - cat(s)
ミルク - miruku - milk
飲みます - nomimasu - (will) drink (polite)
私 - watashi - I/me (polite)
私たち - watashitachi - We/us (polite)
飲みません - nomimasen - will/do not drink (polite)
飲みました - nomimashita - drank (polite)
飲みませんでした - nomimasendeshita - did not drink (polite)
は - wa - topic/subject particle; sometimes translated as "Speaking of..." or "As for..."
一杯 - ippai - a lot (of a liquid); literally "one (cup, spoon etc) full"
太っています - futotteimasu - to get (and remain) fat
を - (w)o - marks a direct object taking a transitive verb; the "w" isn't usually pronounced