Hubo: Pretérito perfecto simple. Implies that is an action finished. Había: Pretérito imperfecto. Action, maybe, continues in present.

There were a lot of people in the room 20 minutes ago? - habia hubo - most time befor

I'll be more simple than my other partners. Hubo: You use this word for things which only happened once and they will never happen again. Había: You use this word when there was/were something but now (there is not/there are not), however, there could be again in other time For example: Hubo un perro en esta casa (Not any more) Había un perro en esta casa (there could be another, maybe) Había muchas personas en mi cuarto esta mañana (There could be a lot of people in other time) Hubo muchas personas en mi cuarto esta mañana (But in this moment not any more) These words are very difficult to explain even for people who speak spanish since they were born. Saludos!

Thank you, your examples really help!

Maybe these examples will be helpful for you: 1. Hubo necesidad de comprar comida..... (tuve necesidad) 2. La gente pensó que él la había golpeado...we are talking about someone 3. No hubo tiempo de poder llamarte (No tuve tiempo) 4. Vine tarde porque habia mucho trafico