Anything vs Nothing, I only have vs I've only, There is vs There are

[Ayan]

Hello everybody,
I'm new to this community and I've found it very useful for English learners so far. Well, I need to ask a few questions that always get me confused while conversing in English. It would be nicer if you give a little bit explanation too, that why blah sentence is correct and why it can't be blah.

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I only have one friend.
I've only one friend.
(Want to say that I don't have more than one friend.)

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I didn't understand anything.
I didn't understand nothing
---
I don't understand anything.
I don't understand nothing.
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There is two girls [in/on/over] here.
There are two girls [in/on/over] here.

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We would all be happy.
We all would be happy.

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I'm off from work today.
I'm off work today.
I'm off the work today.

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Thank you for your effort on reading my post and thanks in advance if you answer any of my queries.

PS: Please correct me if I make any grammar mistake while writing up my questions.

[Hieupt]

Hi, here is my try!

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I only have one friend.
(Want to say that I don't have more than one friend.)

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I didn't understand anything.
---
I don't understand anything.

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There are two girls here.

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We all would be happy.

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I'm off from work today.

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Thank you very much,
Hieu Phan.

P/s: Please just correct if I'm wrong somewhere!