Hi, i have some questions regarding open souce. E.g, theres an open source software, can i use it as a base and modified the software, and sell it? Thanks
It's most likely against the TOS of open source software to modify/change their software, unless for personal use.
The concept of open source software is that developers are welcome to modify and redistribute it publicly as much as they like. If the software is licensed under the GPL, many are, then read here for information on selling modified copies: http://www.gnu.org/licenses/gpl-faq.html#GPLCommercially
It depends on the license that open source software used. For most open source license, revise it and sell it are prohibited. You might want to take a look at en.wikipedia.org/wiki/Open-source_license
In most cases you can sell a modification as a stand alone module/software, but you can't modify an open source software and sell it.
If you want to redistribute OS code, you normally have to include all the source code when you redistribute. Depending on the license, the modifications you made may also become OS.