The main way to set the time in Chinese is through time specification (yesterday, next year…)…
Anyway, if we need to emphasize that action is finished (and distinguish it from other types of actions in the past), we use particle 了 <le> and place it right behind the verb.
<Wǒ zuótiān hē le bù shǎo lǜchá.>
I yesterday to drink le not a little green tea.
YESTERDAY I DRANK QUITE A LOT OF CHINESE TEA.
<Wǒ zài Zhōngguó mǎi le hěnduō Zhōngwén shū.>
I in China to buy le many Chinese books.
I HAVE BOUGHT MANY CHINESE BOOKS IN CHINA.
<Wǒ gěi tā shuō le, kěshì tā bù tīng.>
I to him to say le, but he not to listen.
I TOLD HIM BUT HE WOULDN’T LISTEN.
<Wǒ hē le zhège chá, jiù yào qù shàngkè.>
I to drink le this tea, then yao to go to have a class.
AFTER I FINISH DRINKING THIS TEA, I WILL GO TO HAVE A CLASS.
If action just happened, 了 <le> is placed not after verb, but at the end of a sentense:
<Wǒ mǎi shū le.>
I to buy books le.
I (JUST) BOUGHT SOME BOOKS.
了 <le> (as an indicator of finished action) can´t be used with verbs, that can´t be considered as verbs of a single action, such as…
知道 <zhīdào> TO KNOW
想 <xiǎng> TO THINK
认识 <rènshi> TO KNOW (something, somebody)
是 <shì> TO BE
在 <zài> TO BE somewhere
If we want to say for instance “I used to know how to do this”, we just set it in the past using 以前 <yǐqián> (before):
<Wǒ yǐqián zhīdào zěnme zuò.>
I before to know how to do (it).
I USED TO KNOW HOW TO DO IT BEFORE.
Negation (not just for finished action but for past in general) is created using 没(有) <méiyǒu>, without 了 <le>.
<Wǒ zuótiān méi(yǒu) hē kāfēi.>
I yesterday not to drink coffee.
I DIDN’T DRINK ANY COFFEE YESTERDAY.
<Tā hái méi(yǒu) wèn wǒ.>
He still not to ask I.
HE DIDN’T ASK ME YET.
<Nǐ wèishéme méi(yǒu) gěi wǒ shuō?>
You why not to me to say?
WHY DIDN’T YOU TELL ME?
<Wǒ méi(yǒu) mǎi, yīnwèi tài guì le.>
I not to buy, because (it) (to be) too expensive. (了<le> here is a part of a construction with 太 <tài>.)
I HAVEN’T BOUGHT IT, BECAUSE IT IS TOO EXPENSIVE.
For interrogation, we use the modul 有没有 <yǒumeiyou>, again without 了 <le>.
(There are also other ways, but this one is most common and the most simple.)
<Nǐ yǐqián yǒuméiyǒu zài Zhōngguó gōngzuò?>
You before youmeiyou in China work?
HAVE YOU WORKED IN CHINA BEFORE?
<Nǐ yǒuméiyǒu shōudào wǒ de duǎnxìn?>
You youmeiyou to receive my SMS?
DID YOU RECEIVE MY MESSAGE?
<Tā yǒuméiyǒu gěi nǐ shuō?>
He youmeiyou to you to tell?
DID HE TELL YOU?