[We are permitted to anser only one question per post. My explanaltion will help you to do the others.]

The main differences between active and passive voice are (1) in the involvement of the subject regarding the  state or action of the predicate (verb).  That is, in passive voice the subject of the sentence is NOT the doer of the state of existence or action of the verb, whereas in active voice the subject is clearly the initiator, or doer, of the verb's state of existence or action. And, (2) the formation of the predicate.  In the predicate of a passive voice sentence there is always a verb phrase formed with the verb to be and the past participle.

In each of the above sentences, for instance, the subjects are being acted upon; they are not the initiators of the action.  So, in order to change them to active voice, the sentences must be turned around so that the subjects become the doers of the action.  In addition, the predicate must be changed from the passive form which is the use of a tense of the vert to be + the past participle (e.g. was seen) to the active form of the verb (e.g. saw) Note that it is important not to change the tense of the predicate when going from passive to active voice.

Here are the changes:

1. The doctors at the hospital are treating Jane. (Alternatively: The hospital is treating Jane.)

Remember, too, that the use of active voice is preferable to passive since it clarifies meaning (English speakers are used to hearing/seeing subjects early in sentences) and prevents complications of structure along with wordiness.