However is the king of contrast. We use it every time we want to say something different from what we said before or call people's attention, however, use "however" with moderation: too much " howevers" can make your presentation very boring. Therefore, if you don't want people to feel bored...ops...I used " therefore" already without introducing it properly. I do apologize for that.
Well, "therefore" is our second special word. We use " therefore" every time we want to conclude a topic or anticipate a climax. If "however" contrasts; "therefore" explains, defines and gives people insights about the topics, themes, issues or subjects that we are talking about.
However, don't use those two words right way. Be patient! Only with patience you will embody those two exciting words, therefore, feel them as part of your vocabulary before trying to use them indiscriminately.
However (have I said " however" again?), people who use " however" and " therefore" in their presentation are considered to be very sexy, specially because their audience feels turned on.
If you don't believe it, use " however" and " therefore" in your next presentation and you will see that my forecast will be very accurate.