Q&A for How to Use However

The simplest answer is that "could" is the past tense of "can," and "would " is the past tense of "will" or "shall." It's more involved than that, however, and your best option is to look up those words in a dictionary.

"Hence" means "therefore": "The situation scared me; hence the weapon I carried." It also means "in the future": "It occurred three years hence" (three years from now).

They are similar but not identical in meaning and thus are not interchangeable. One difference is that you can start a sentence with "however" but not with "but." (There are other differences, too.)

Yes, place commas before and after "however." This should read as: "It has, however, been observed that..."