nai-form is used in present-negative sentences for Plain-form. For example, "don't write", "don't eat", "don't come", "don't study". Hope this explanation clears your doubt about the difference between the two forms.

How do you use NAI in Japanese?

You first change KI to KA, and add NAI. So, the NAI form of IKIMASU is IKANAI (not to go). But after you drop MASU, if the syllable just before MASU is the vowel "I" and it is not attached to a consonant, you change "I" to WA, and add NAI.

How do you use Nai De?

The first use is to say “Please do not [verb].” You can say “Please do not [verb]” either by using ～ないで ~nai de as it is, or by adding ください kudasai (please) to the end (the former can come across as quite casual; the latter comes across as more on the polite side).