Also, select & copy the 'Youtube' url you want, then paste into: www.videogetting.com and just follow instuctions.
I don't think the 'Youtube' police will be battering your door at three in the morning!!
Technically, yes you can. Wideboy's link is excellent.
Legally, you may not be able to. If they say you are free to use their work, then you can (under their copyright terms). Otherwise, no you can't, not without their permission.
Depending on what country you're in, you may be able to use fair usage laws which will allow you to use a few seconds' material.