One conjunction for two clauses
In English, we use just one conjunction to connect two clauses.
Incorrect: Because he is intelligent so he gets good marks.
Correct: Because he is intelligent he gets good marks. OR He is intelligent so he gets good marks.
Because is a conjunction and English does not require a second conjunction.
Incorrect: Since he was angry therefore I said nothing.
Correct: Since he was angry I said nothing. OR He was angry; therefore, I said nothing.
Since is a conjunction and it is enough to join the two clauses: He was angry and I said nothing.
Therefore is not a conjunction. It cannot connect two clauses. It is a transitional adverb. A transitional adverb should be separated from the rest of the sentence with a comma.
Incorrect: He did not come to work. Because he was ill.
Correct: He did not come to work because he was ill.
A subordinate clause cannot stand alone. It must be attached to an independent clause.
However, there are some exceptions to this rule. In natural spoken English, because clauses can stand alone.
‘Why are you laughing?’ ‘Because you look funny.’ (More natural than ‘I am laughing because you look funny’.)
Sentences beginning with a negative word
Incorrect: Neither he comes nor he writes.
Correct: Neither does he come nor does he write. (Formal)
Correct: He neither comes nor writes. (Informal)
When a negative word (e.g. neither, hardly, seldom, never, scarcely etc.) comes at the beginning of a sentence, the main verb must be inverted, as in a direct question. As you know, the verb comes before the subject in direct questions.
If there is no auxiliary verb, we use a form of do.
Another example is given below.
Incorrect: Neither he smokes nor he drinks.
Correct: Neither does he smoke nor does he drink.
Correct: He neither smokes nor drinks.