Why does the Torah here write "Vayarev ha'Am", whereas earlier (in 15:24 & 16:1) it writes "Vayilonu ha'Am"?
Ramban: Whenever the Torah uses the latter expression it implies that Yisrael merely complained about their situation - "What shall we do?", "What shall we eat"? or 'What shall we drink?" Whereas the former is used when they made demands of Moshe; "Give us water, you and Aharon your brother!"
Then why, in the following Pasuk, does the Torah switch to "Vayalen ha'Am"?
Ramban #1 (citing Ibn Ezra) and Targum Yonasan 1 : There were two groups, One came to strive with Moshe, the other, to test Hashem.
Ramban #2: When they began to make demands of Moshe he immediately pointed out that Hashem was in charge and that it was to Him that they ought to turn. 2 So they withdrew their demands of Moshe and remained silent for a day or two until the water in their Keilim ran out, when they complained (Vayalen) that they had nothing to drink (in their usual vociferous manner, but without demanding from Moshe).
Who translates "Vayarev ha'Am" as "the Resha'im among the people strove", and "Vayalen ha'Am" as "the people complained".
Indeed, their intention was to test Hashem - whether He was able to provide them with water, as the Torah testifies in Pasuk 7), and Moshe immediately realized that (Ramban).