1. Adverbial clauses utilize some specific conjunctions like:
- ... in order + infinitive
- ... as [soon, far, long, etc.] as ...
- ... if ...
- ... unless ...
- ... as ...
- ... after ...
In this instant it seems best to use in order + infinitive to show that the subject had done something, so he could have achieved something else, hence the changed sentence will become:
' He has been working very hard in order to clear the entrance examination '
2. Second sentence is a bit more straightforward as we have only one option. Too, used inside a sentence can express that a subject is experiencing more of a particular feeling, emotion, state of mind or body to perform the action stated in the sentence. In other words, whenever you want to say that you cannot do something, because you are tired, worn out, exhausted, occupied, etc. you will use the following contruction::
N(oun) + to be + too + ADJ(ective) + to + V(erb)
In this situation, the sentence will be:
I am too tired to work anymore
I hope that this explanation will help you understand the problem better and empower you to solve those kind of questions without any problems from now on :)
Your neighbourly Admin