Only those adjectives which end in ा in their standard (masculine singular) form change to ी for a feminine subject and े for a masculine plural subject. Eg: बड़ा(big), थोड़ा (some) , काला (black) etc.
Other adjectives do not change according to gender and number. Eg: जवान (youthful), लाल (red), दुखी (sad) etc.
There are some exceptions to the rule though. Eg: ज़िंदा (alive) which doesn't change form.
It's due to the phenomenon of Schwa deletion in Hindi.
The basic form of every consonant is the consonant sound attached to the vowel अ. Therefore, त is 'ta'. But in Hindi, this implicit vowel is never pronounced when it is present at the end of a word. So, सेब is 'seb' (not seba), आप is 'aap' (not aapa) etc.
When in the middle of a word, the implicit अ vowel is sometimes pronounced and sometimes not. For example, जानवर is pronounced 'jaanvar' which means the vowel that is tagged on to व is pronounced but not the one tagged on to न (and of course also र because it is at the end of the word). The Wikipedia article I've linked to above goes over how you can guess if the अ should be pronounced or dropped.