話せます hanasemasu - can speak becomes...
私は、日本語が話せません。
watashi wa nihongo ga hanasemasen.
(I) can`t speak Japanese.
分かります wakarimasu - understand becomes...
私は、英語が分かりません。
watashi wa eigoga wakarimasen.
(I) don`t understand English.
If you can make the -masu form, just drop the す and add the せん。
You may have noticed there are no `no` words needed to make a negative like in English. You simply modify the verb`s ending.
To make the negative in the plain, or simple, form by taking the basic stem and adding ない to it.
With the `ru` verbs you simply drop the
る and add ない as in 忘れる wasureru (to forget)...
日本語を忘れない。
nihongo o wasurenai.
(I) don`t forget Japanese.
And for the `u` verbs we change the ending
`u` sound to a `a` sound as in 書く kaku -> 書か...
手紙を書かない。
tegami o kakanai.
(I) don`t write letters.
Finally we come to する and the other irregular verbs