Slightly mundane question here. Do negative verbs translate as x "does not" do this action or simply "not" e.g 食べない "not eat" as opposed to "does not eat". The reason I ask is because On Tae kim's site all the verb examples are translated as "does not", When suddenly 買わない becomes "not buy" - which is it? I know it's a silly a question, but I'm pretty curious, so I'd really appreciate any answers.
In English, the auxiliary verb do is a meaningless verb that is inserted for various reasons. For example, direct questions require an auxiliary verb in modern English:
1a. *Like you the movie?
1b. Do you like the movie?
When you form an English question like this, the subject and auxiliary swap places. But the lexical verb like isn't an auxiliary, so example 1a is ungrammatical. You insert do so that you have an auxiliary to switch with the subject.
And in modern English, you need an auxiliary to add the negative not or negative affix -n't:
2a. *She takes not the money. / *She takesn't the money.
2b. She does not take the money. / She doesn't take the money.
The examples in 2a don't work because take is a lexical verb. In this kind of English sentence, you can only negate an auxiliary verb, so you insert the meaningless verb do.
Japanese is completely different. It has a negator
-(a)nai which attaches directly to verb stems. There's no need to insert anything like do, so whether or not you translate it with do depends on what's required in English. It has nothing to do with Japanese.
To form the negative in Japanese, attach
-(a)nai to the stem of a verb:
食べる tabe-ru 食べない tabe-nai
tabe- ends in a vowel, so you add
-nai without the
話す hanas-u 話さない hanas-anai
hanas- ends in a consonant, so you add
-anai with the
(a). Adding this extra vowel prevents you from saying the consonants /s/ and /n/ with nothing in-between, which Japanese doesn't allow.
Your example 買う is a consonant stem verb, too, even though it doesn't look like one. The stem is
kaw-, so you insert the
買う kaw-u 買わない kaw-anai
The reason this seems weird is that /w/ disappears before every vowel except /a/. So the /w/ disappears from
kaw-u, and you're left with
The main exception to this pattern is ある, which becomes ない instead of *あらない. And there are other negators (such as ん in ません, used to form a polite negative). But I'm limiting this discussion to ない, since that's the negator you asked about.
As you can see, there's nothing corresponding to do in these Japanese examples. Whether you translate these with do or not depends entirely on the requirements of English, and has nothing to do with the Japanese.
Tae Kim sometimes uses over-literal translations. I'm guessing the one that confused you was "As for me, not buy" (私は買わない).
Without context, this could mean:
1) A specific statement about the future: I'm not going to buy/I won't buy (thing we were talking about)
2) A more general statement about your habits: I don't buy (thing we were talking about)