When is the subjunctive mood used? I don't understand the phrase "Que tenga un buen día." Isn't something missing in this sentence?
The subjunctive is used when something has not actually happened yet. It may or may not happen. For example, "I hope you have a good day." "[Espero] que tengas un buen día." Here the speaker just "hopes" the other person will have a good day. Since it hasn't happened yet, subjunctive is needed. Notice that the first verb in this sentence is normal present tense. "Que tenga un buen día" is a subjunctive clause where the main verb is implied rather than stated explicitly: [Espero] que tengas un buen día. [I hope that you] have a good day. We do the same thing in English with this same phrase, leaving off "I hope that you," but in Spanish the phrase would sound like a command if the word "Que" were not there at the beginning.
How do I add the opposing viewpoint into my thesis statement?
One way you can do this is to begin your thesis statement by saying something like "Although many people feel that . . . ." and then go on to tell your argument. For example, "Although many are opposed to abortion and believe a woman should be able to decide what happens in her own body, a fetus is a living being that feels pain, is constantly learning, and should have the same rights as any other human being."
When do I use "then" and when do I use "than"? Does it even matter? Do they both mean pretty much the same thing?
"Then" and "than" are not equivalent terms, but are used in different situations. Use "then" when you are writing about when things happened in time order. For example, "I clipped my dog's coat, and then I gave him a bath." Use "than" when comparing two things. For example, "Wild-crafted herbs are better than organic herbs."