Use of "lo"

Hi,

Can someone help with some advice please?

I keep seeing “lo” in Italian sentences, where it is not used in English. I would like to know if:

1. 1. “Lo” is necessary 
2. 2. If we only use “lo” with specific verbs
3. 3. If there are any rules about when to use “lo”

I have some examples:

Non lo so - I don't know (previous fact). In English we don't need to say ‘it’.

Lo sai una cosa - Do you know something. Again, in English we leave out “it”

Non me lo ricordo - I don't remember (previous fact)

Karen non lo vuoi dire - Karen doesn't want to say (her age)

Thanks!