The subject is the thing that 'does the doing' in the sentence. In this case, the students are the subject. The verb is the action that done by the subject, so in this case, the students watched. The direct object is the objects that the verb is referring to in the sentence, so in this case, the direct object is video. There is no indirect object, or the person or thing that the verb is done for.
Just watch this video it will help https://www.khanacademy.org/math/algebra/introduction-to-algebra/alg1-manipulating-expressions/v/combining-like-terms-and-the-distributive-property
