"Yo" doesn't have function to make a question. "Suki da yo" is an informal form of "Suki desu yo". You can put "yo" regardless the sentence is formal or informal, positive or negative, present or past. The usage is very simple. With putting "yo", the sentence becomes more interactive. ex) Iku. (I go) This is just describe the situation. Iku-yo. (I will come, or Let's go) In this sentence I'm talking to other person. If the person is near, the meaning is " I'm going, you should also prepare to come with me" If I'm talking on the phone, the meaning is " I'm now coming!" By putting 'yo', it becomes clearer that I don't talk to myself, but to you, and require your response.

"Suki" is technically an adjective, a na-adjective to be exact. In order to make a sentence with a na-adjective, a copula ("to be"-verb) is normally added. Sometimes the copulas "da" and "desu" mean "be" but sometimes they are just added to complete a sentence or in the case of "desu," to make speech polite. Also, be careful when using "da," and "yo." "Da," is informal and you may insult someone by using it if you are not a close friend. When in doubt, "desu," is always the way to go. Putting in "yo" at the wrong time can make you sound too aggressive. It is often used to stress things or introduce new information. Also when using it in speech, don't vocally stress it, pronounce it with a lowering intonation otherwise, you may sound rash and aggressive.