1. This is true. "Infinitive" simply indicates a verb that hasn't been conjugated. For example, "to walk" is an infinitive verb, while "I walk" is conjugated. To denote the infinitive in English, the word that comes before the verb is "to." On the other hand, the infinitive is indicated in Spanish by the endings, because Spanish verbs are conjugated according to their endings: "Caminar" means to walk (infinitive), but "Yo camino" is I walk. 2. This is true. To negate something in Spanish, you will most often put the word "no" before the verb: "Yo camino" then becomes "Yo no camino" (I don't walk). 3. To agree with someone who likes something, you will use the word "también." To agree with someone who dislikes something, you will use the word "tampoco." The best way to think of these words is también as "me too" and tampoco as "me neither." 4. If you don't want to use either of these words, you can simply say "de acuerdo," which means I agree.