These connectors can be placed in the middle or at the beginning of the sentence. A comma is necessary when the sentence starts with the linker, so that the main and the subordinate clauses are graphically separated.

What does the keyword in ensure during a loop over a list?

They are linkers that connect 2 separate paragraphs, or two separate sentences. They are preceeded and followed by a punctuation mark (typically comma, or full stop).

She's still very popular...... of her bad behaviour.

He's usually very kind. On ...... hand, he can be moody.

I wouldn't go, .... I had enough money. I hate the place.