Certaines vs certainnes - abnormal spelling?

Question

Is certaines different to many French adjectives in it doesn't go to -nne in the feminine.I spelt it certainnes

Accepted Answer

Nigel, 
Ending in ‘ n ‘ alone is not the criterion for taking ‘ - nne ‘ ending to transform adjectives from masculine to feminine spelling. 
The vowels preceding the terminal ‘ n ‘ in the masculine form generally indicate whether the ‘ double n ‘ comes into play. 
The last 2 links below discuss particular situations : 
 https://progress.lawlessfrench.com/revision/grammar/how-adjectives-usually-take-an-e-in-the-feminine-form-feminine-adjectives 
https://french.kwiziq.com/revision/grammar/how-to-form-the-feminine-of-nouns-and-adjectives-ending-in-ien-ion-and-on-adjectives-nouns
http://www.french-linguistics.co.uk/grammar/adjectives_form_n_doubling.shtml