了 — and why it is not a past tense

규칙 了 is NOT a past tense. 我每天六点起床 stays 了-less even when it is about yesterday.
了 after the verb: the action completed — 我买了一本书。
了 at the end: the situation has changed — 下雨了。 现在几点了？
The negative is 没(有) + verb, and the 了 GOES: 我没买书。 Never 没买了。