Position of adverbs

Here are the main rules and exceptions on the position of adverbs in sentences:

First rule: if there is only one verb in the sentence, the adverb comes before the verb as in the following example: He usually comes to work on time.
Exception to the first rule: if the only verb in the sentence is 'to be', the adverb comes after the verb as in the following example:  He is usually on time.

Second rule: If there is more than one verb in the sentence, the adverb comes after the first verb as in the following example: He has already arrived at work.
First exception to the second rule: if the first verb in the sentence is an auxiliary verb with 'to', i.e., have to, need to, ought to and used to, the adverb comes before the first verb as in the following example: He usually has to start working by 9:30 AM.
Second exception to the second rule: if the first verb in the sentence is a contracted negation, the adverbs 'probably', 'possibly' and 'certainly' come before the first verb as in the following example: He probably won't come to work on time.

These being the main rules and exceptions, there is more to this topic.  You can find more information on this webpage, which also provides you with some exercises.