This is the 2856th most frequent Arabic word.
لست
"لست" means "I am not."
In this sentence, 'لستُ' is used to express the negation of existence or presence, as 'I am not.'
لستُ هنا لأنني أردت ذلك.
I am not here because I wanted to be.
The word 'لستِ' here is used to negate being alone for the addressed female listener ('You are not').
لستِ وحدك في التفكير بهذا الشكل.
You are not alone in thinking this way.
In this sentence, 'لستَ' is utilized to express negation of requiring assistance for the addressed male listener ('You do not').