-ㄹ/을 텐데 is short for -ㄹ/을 터인데. 터 is a dependent noun that means one's plan(the action is going to be taken), guess/expectation, will. 피곤할 텐데 좀 자. You would be tired, get some sleep. 너도 같이 가면 좋을 텐데. It would be good if you go with us. 곧 갈 테니까 조금만 기다려. I will get there soon, so please wait a minute. I'd use '-ㄹ/을 것 같아요' when I say my thoughts gained from my observation. It's also used as a weaker one for "I think...". We tend to avoid making an assertive statement, like you don't say "Is it delicious?" I'd use -아/어 보여요 when I talk about something I'm seeing now. "also, using 보여요 as a helper verb meaning to seem like soandso would be used like this, right? 피곤해 보인데.. you seem tired?" Yes.

well, i think, you're good at korean. anyway. the differences between those words is hard to explain. so i'll make some sentences. 1. If you cut your hair, it looks more gentle.- 를/을 것 같다. 2 If you cut your hair, it looks more gentle.- 를/을 텐데. it souns same but it's nuance is different. the first sentence is like an advice. and the other is feeling of sorry. exactly, at first sentence, the telling person talks to another how about cutting hair and the other sentence means ' oh, did you cut your hair? i think if you cut your hair, you look better' and, ㄹ/을 것 같아요 is suposition. 보여요 is the real scene what u see. please leave a massage if u don't understand