1- We use : Though / although / even though at the beginning or in the middle of the sentence to join two independent sentences (usually), the surprising sentence comes in the independent clause.
Example : Even though I did my best (not surprising) , I failed to pass that test. (surprising)
2- One of many uses of "but" is to join two independent clauses, in which case it is followed by the surprising part of th>
Example : I did my best , but I failed to pass that test.
3- We use "however" in many different ways:-
a) it can be preceded by a semi-colon or a period and join two sentences as follows:-
I did my best;however, I failed to pass the test.
b) It can be preceded by a comma + and" as follows.
I did my best, and however, i failed to pass that test.
c) It can be placed in mid or final position in the second sentence preceded and followed by a comma as follows:-
I did my best. i failed to pass that test, however.
or : Ali is clever. he, however, never studies.