"They do not understand."
Actually, chinese words that are composed of mor than 1 character tend to stress one of its characters. In this case, the stress is in the first character, so, unless you say the word really slowly and clear, this tone would be pronunced as a 0 tone. Here is another example:
认识 rènshi ren4shi0
It's not a falling tone, it's no tone at all. The second syllable is unpronounced like in 妈妈 ma1ma.
I suggest using an app like Pleco to make sure the pronounciation is right.