Translation:Please don't go to that mountain in the winter.
In negative conjugations, a godan verb with a く ending changes the く to か before adding the ない ending. So 行く (to go) -> 行かない (to not go), 歩く (to walk) -> 歩かない (to not walk), and so on. All verbs in the godan verb group conjugate to the negative form in this way with the "u" sound changing to "a." So for example:
飲む -> 飲まない
泳ぐ -> 泳がない
遊ぶ -> 遊ばない
死ぬ -> 死なない
待つ -> 待たない
Well... you probably get it. The one difference is う ending godan verbs which change the う to わ, so 会う -> 会わない, not 会あない. ある is also the only godan verb with an irregular negative form, as it becomes ない, as opposed to あらない.
Thats just the way the conjugation works for this verb ending (negative plain form + で). This works differently for the two main types of verbs (るverbs and うverbs). I woukd definitely look this up for a better understanding of the difference. For this conjugation, you take the final る off of the る verb and add ない. So, 食べる-->食べない. For うverbs, you switch the う for its corresponding あ sound and add ない. So, 読む-->読まない; うたう-->うたわない; etc. By the way, the two irregulars work as follows: 来る-->こない; する-->しない
In my opinion, ください should translate to please, because the gap of politeness between e.g. "Eat it!" and "please eat it" is big enough. Not having "please" has a poorer translation quality. I cannot say it is wrong but it is just not as good.
To translate "Do not go" in imperative form, there are quite a few options: