Make sure (that) you drink lots of water.
(Is 'sure' an object complement? 'you drink lots of water' is an object, isn't it? Am I right? Can we use 'that' in the sentence?)
Sorry, but I do not understand your question or questions. I don't even see where you would like to use "that".
Sra
In the sentence "Make sure (that) you drink lots of water," the word "sure" is not functioning as an object complement. Instead, it is an adjective modifying the verb "make." The object of the sentence is "you drink lots of water," which functions as the direct object.
In this sentence, the word "that" is optional. It can be used before the subordinate clause "you drink lots of water," but it can also be omitted without significantly changing the meaning of the sentence. Both of these versions are grammatically correct:
1. Make sure you drink lots of water.
2. Make sure that you drink lots of water.
The presence or absence of "that" does not affect the sentence structure or the role of the words within it.