Can past tense come after present lexical verb? Is it correct to put past tense at the back of 'ARE', e.g "are clothes sold cheaply over there", or "we are not paid yet"?
Yes you can, but you have to be careful as it doesn't always work nicely. Your second example, we are not paid yet makes sense, but it is not overly natural. You'd more likely hear someone say we have not been paid yet.
However, if, for example, it was said by two interns, it would make sense because it would suggest that they have not yet been added to the payroll.
So for a learner, there are nuances that have to be understood in order to use the present perfect simple.
You question is about the passive tense in English.
- Clothes are sold over there [by clothes sellers]
- We are paid in euros every week. [by our employers]
- He is paid in francs CFA once a month. [by his boss]
Passive sentences in the present tense are formed using is for a singular subject and are for a plural subject and a past participle (the "third column" of verbs)
However, if you have not received your wages, you would say:
We have not been paid [by our employers] this week. [a passive in the present perfect tense]. Examples: She has not been seen for a week [by us], They have not been scolded [by their parents].
Passive construction can be present [is/are paid], present continuous [is/are being paid], past [was/were paid], past continuous [was/were being paid] etc. This is not the whole story by any means. :)