Learn Spanish (Latin America): Habría un seminario interesante; sin embargo, nadie podría asistir. - There would be an interesting seminar; however, no one would be able to attend.

Habría comes from the verb haber (to have / to exist). In the conditional tense, haber has an irregular stem: you drop the e from the infinitive to get habr-, and then add the regular conditional ending -ía. Just like hay is the special form for "there is", habría is the form used for "there would be".

How does poder change to become podría?

Just like haber, the verb poder (to be able to) has an irregular stem in the conditional tense. You drop the e from the infinitive ending to get the stem podr-, then add the ending -ía to mean "would be able to" or "could".

Doesn't asistir mean "to assist"?

It looks exactly like it, but asistir is a classic false friend! It actually means "to attend" (like attending a seminar, a meeting, or a class). If you want to say "to assist" or "to help", you would use the verb ayudar.

If nadie means "no one", why isn't there a no before the verb?