Or HaChaim on Numbers 9:18/אור החיים על במדבר ט׳:י״ח

1 על פי ה׳ יסעו בני ישראל, the children of Israel journeyed at the command of G'd. The cloud was the means by which G'd made His wishes in the matter known.

2כל ימי אשר ישכון, all the days that the cloud would remain stationary, etc. Our verse details the various ways in which the cloud behaved. It also pays a compliment to the Israelites who made their own wishes correspond to G'd's wishes. The verse first tells about instances when the cloud remained stationary for a long period by stating that whenever the cloud remained in place for a long time the Israelites "were willing to remain encamped in that location during the entire length of time that the cloud showed no sign of moving." The proof for this is that the Torah speaks of יחנו in the future tense instead of reporting חנו, "they did remain in camp" in the past tense. The Torah continues by saying ובהאריך הענן, "and when the cloud tarried, etc." (verse 19), to tell us that even if the cloud remained stationary for an inordinately long period of time the Israelites did not demur but were content to stay where they were. The verse goes on to say: ושמרו בני ישראל את משמרת ה׳ ולא יסעו, "the children of Israel observed the orders of G'd and would not journey." This shows that even though there may have been times when the Israelites- had it been up to their own volition alone-would have liked to move on, they resisted such a temptation and voluntarily abided by G'd's wishes in the matter. This is why the Torah wrote ולא יסעו, "and they would not journey" instead of writing ולא נסעו, "and they did not journey," to make certain we would not interpret their remaining in a place for a long time as their first choice.