Assalaamu alaykum wa rahmatullahi wa barakatuh,
Can I ask, how would you know when to do ghunnah for double kasra, double thumma and double fatha?
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
The "double" fathah, dhammah, kasrah, is a symbol for the vowel plus an extra noon saakinah (non-vowled noon). This is called a tanween. The rules of noon as-saakinah and tanween help us determine how we pronounce -or don't pronounce- the noon saakinah or tanween depending on the letter which follows. To learn the rules of the noon saakinah and tanween, please see the following link: