Is it true we existed before birth? If so how? In what form. Please clarify
If you are referring to our souls, then yes. Allah s.w.t. created everything and that includes our souls, the form of ourselves which is our identity that transcend through each life including this life of dunya (worldly life).
Our souls pre-resided in a place called alamulmarwa (the universe of the souls), before being sent into the wombs of our mothers with only one memory, which is the knowledge we have of our creator.
Here's a brief list of our souls' journey:
- The Universe Of The Souls/'alam al-marwa(approx. in multitudes of 100k years)
- The womb of our mothers(approx. 9 months)
- The Worldly Life / Ad-Dunya (approx. 60 years in the modern age)
- The Grave / Al-Barzakh (approx. in a few hours or hundreds of years)
- The Resurrection / Yaum al-Qiyamah (approx. 3,000 to 50,000 years)
- Akhirah / Hereafter / Heaven or Hell (infinite)
I'm not an Islamic scholar so please find much better and complete answers than what I have put forth. You may also watch this video, they provide a little bit of reference: https://www.youtube.com/watch?v=y58jT-YgdP0
and really, Allah s.w.t. knows best.