A conjunction is a word which joins two or more than two words, phrases, clauses or sentences.

Types of Conjunctions

1. Coordinating conjunctions : for, and, nor, but, or, yet, so etc.

2. Subordinating conjunctions : after, although, as, as if, as long as, because, before, even if, as though, even though, if, if only, so that, than, rather than, since, though, unless, until, when, whenever, where, Whereas, while, etc.

3. Correlative conjunctions : either or, neither nor, both and whether or, not only but also.

Important Rules to use of Conjunctions

1. The conjunctions ‘and’, ‘but’ and ‘or’ are used to join different types of sentences.
(i) The word ‘and’ joins two clauses of same nature.
(ii) The word ‘but’ joins two antithetical clauses.
(iii) The word ‘or’ express an alternative or choice.

e.g., (i) The man is poor but he is honest.
(ii) Naresh is brilliant and Shalini has a pleasant smile.

2. No sooner is followed by than and not by when.
e.g., N o sooner had I started than the rain started.

3. As, Because and since are used to express reasons.
e.g., As I was walking down the street I saw Mohan driving a car.
‘Because’ is used when the reason in the most important while ‘as’ and since are used when reason is less important.

4. Either-or, Neither-nor, not only-but also, bothand, whether-or etc. should be followed by the same parts of speech or of the same function.
(i) I am neither happy nor excited.
(ii) The monkey ate both the banana and the bread.

5. Scarcely and Hardly should be followed by when.
e.g., Hardly had he arrived when he had to leave again.

6. Unless, until, if not, so that should not be followed by not.
e. g.,
(i) I will wait until he comes.
(ii) You will not succeed unless you work hard.

7. When suppose is used in the beginning of a sentence to denote a command or a request, it is not followed by if.
e.g., Suppose (if) you are caught, what will happen to your parents.

8. After the adjective other, than should be used for contrasting one thing with another. (The use of prepositions from, but, except in such a connection is wrong)
e.g., He had no other option than to sacrifice.

9. ‘However’ and ‘But’ should never be used together in a sentence.
e.g., (i) But that however is not repairable (Incorrect).
(ii) But that is not repairable (correct).

10. The use of present tense after as if and as though should be avoided. Past perfect or past tense with a present meaning is used after these conjunction.
(i) He looks as if he suspected something.
(ii) He seemed as though he hadn’t had a nice meal for many days.

11. Such as is used to denote a category whereas such that emphasizes the degree of something by mentioning its consequences.
e.g., . (i) Sachin played such an innings as played by the best batsman.
(ii) Sachin played such an innings that it took the match away from the opposition.

12. Until is used in reference to ‘time’ while unless refers to a condition.
e.g., (i) I will stay here until you return.
(ii) You cannot achieve your goal unless you try for that.

13. (a) After some verbs such as believe, hope, suppose, think, presume, afraid of etc. ‘that’ can be omitted .
e.g., (i) I think he will come.
(ii) We presence you are innocent.

(b) Agree, assert, calculate, conceive, maintain, state, suggest, assume, avert, learn, understand. These are some verbs after which we can’t omit ‘that’.
(i) I agree that your proposal is very good. 
(ii) He suggested that we should go there.

14. The conjunctions though (or although) and but do the work of setting one statement against  another by way of opposition or contrast and therefore the correlative of though is yet or a comma (,) will used.
(i) Though he is poor, he is honest.
(ii) Although he worked hard, yet he could not top the class.

15. If a not only ...but also sentence, the verb should agree with the noun or pronoun mentioned second, because this is the part being emphasized.
e.g., Not only the teacher but also the students are enjoying themselves.

16. Both is followed by ‘and’ and not by ‘as well as’_ besides both has positive sense and cannot be used in negative sentences.
She is both intelligent and hard working.

17 . ‘Nor’ is commonly not used as other conjunctions It is commonly used in the correlative pair.

18. Should is used after lest. ‘Not’ is not used after lest. e.g., Word hard lest you should fail.

19. After ‘because’, ‘since’, ‘as’, ‘as soon’ as, we should not use therefore and thus.
e.g., As soon as I reached the station, the train started.

20. Relative pronoun ‘that’ or ‘as’ is used after ‘the same’.
e.g., She is the same girl that met me yesterday.

21. ‘Or’ is used after not and never.
e.g., We have never met her or written to her.