How Do You Say " I Understand A Little Bit " In Japanese?

It's on this site/page (very good for learning Japanese!):

Http://web-japan.org/kidsweb/language/lesson3/lesson3_01.html
http://web-japan.org/kidsweb/language/lesson3/lesson3_02.html
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If someone asks you if you understand Japanese, you'd say:
Hai, chotto wakarimasu. - Yes, (I) understand a bit.

If you want to say 'I understand a little Japanese', it's :
Nihongo ga chotto wakarimasu.

(both these are on the site I've given you.)
Hope this helps