1. ísimo, a, os, as : adding a sense of very
:extremely, super, very, exceptionally
emphasizes the adj.

2. Las reglas grandísimo grande + ísimo
1. If the adjective ends in a vowel : drop the vowel before adding the ending:
grande ísimo
grandísimo

3. Las reglas malísimo mal + ísimo
2. If the adjective ends in a consonant : simply add the ending to the singular form
mal ísimo
malísimo

4. Spelling Changes c g qu gu c occurs when the adjective ends with :
c, g or z
(after taking off end vowel) c g z qu gu c

5. Spelling Changes rico largo feroz riquísimo larguísimo ferocísimo
occurs when the adjective ends with :
c, g or z (sometimes after taking off end vowel)
rico
largo
feroz
riquísimo
larguísimo
ferocísimo

6. Make sure the ending of ísimo agrees with the noun it is describing.
ísimo/a/os/as
Las montañas son altísimas.

7. Accents
All other accents will drop.
fácil  facilísimo
The tilde (ñ) will stay.
pequeñopequeñísimo