Mary's answers above are correct, but let me give some more explanation: 1) The correct verb is "use" and not "uses," in order to maintain parallel sentence structure. This is indirectly related to the use of "would," an auxiliary verb, in the sentence. It would keep ___ and only use the system to ___... In the above example, the auxiliary verb "would" applies equally to both verbs - think about breaking the sentence into two parallel parts: It would keep ___. It would only use the system to ___. [correct] It would keep ___. It would only uses the system to ___. [incorrect, non-parallel sentence structure] However, if the auxiliary verb is not necessary in the sentence (depends on your intended meaning), then consider the following: It keeps ___ and only uses the system to ___... Now, in this example, the two verbs (to keep and to use) need to be parallel with each other and also match with the noun ("it"). Again, think about breaking the sentence into two parallel parts: It keeps ___. It only uses the system to ___. [correct] It keep ___. It only uses the system to ___. [incorrect, wrong verb conjugation in first sentence] It keeps ___. It only use the system to ___. [incorrect, wrong verb conjugation in second sentence] 2) Yes, Mary is correct - the sentence should be "You must leave at the right time."

U should use "use" With would

You must leave at the right time..