A direct object is the object that the action affects/applies to: in "I saw it/him/her," "it/him/her" is the direct object. An indirect object, in its expanded form, requires a preposition and has a direct object next to it. An indirect object is the receiver of the direct object: in "I gave the present to him," "to" is the preposition, "the present" is the direct object, and "him" is the indirect object. The shortened version would be "I gave him the present;" notice that there is no more preposition here. Hope this helps.
Thank you again. It is not the sentence that is "indirect" or "direct;" the sentence contains both one indirect object and one direct object. The indirect object is "him," and the direct object is "the present." Normally (I want to say all the time), when there is an indirect object, there must be a direct object. Notice you cannot say simply, "I gave (to) him," because it feels like something is missing. What did you give him? That "something" that is missing is the direct object. However, you can say, "I called him," because "him" here is a direct object. Indirect objects normally cannot stand alone; they need a direct object nearby in the same sentence.
For most, if not all examples of the imperfect, you can use "used to," because there is no context. We do not know what else is going on other than the sentence Duolingo is giving us. Some sentences, though, Duolingo will not accept "used to." Your best bet is to use "was/were" + the verb "-ing." For example, for this sentence, you would say, "We were seeing him every Saturday." Although this sentence sounds awkward here, it can be used: "At that difficult period of time, we were seeing him every Saturday." I guess it is best to use your best judgment, but I have noticed that Duolingo normally accepts the "used to" and the "was/were" + "-ing" construction the most.
If you are talking about English grammar in general, "used to" is used when an action happened frequently in the past but not anymore. "I used to go to school Monday through Friday," means there was a time in the past when I would always go to school from Monday through Friday, but now I do not anymore. Hope this helps. :)