Adverbs usually modify a verb or a verb phrase. A sentence adverb is an adverb which does not merely modify a verb or a verb phrase, but which instead modifies the entire sentence containing it.
A sentence adverb usually expresses ideas such as the probability, desirability or other characteristic of the situation described by the rest of the sentence.
Examples of sentence adverbs are: probably, surely, certainly, undoubtedly, frankly and hopefully.
He will probably win. (= It is probable that he will win.)
He will certainly win. (= It is certain that he will win.)
Hopefully he will win. (= I hope and expect that he will win.)
Surely he will win. (= I can’t believe that he will not come.)
Sequence of tenses
The choice of tense in a subordinate clause results from the choice of tense in a preceding main clause. For example, if the verb in the main clause is in the past tense, the verb in the subordinate clause too will be in the past tense. Example: James said that he was interested in the offer. Here the verb in the main clause (James said) is in the past tense. Therefore we put the verb in the subordinate clause into the past as well.