Here are a few correct sentences: "Could we start the work in June, since all of the members prefer that?" "Could we start the work in June, since all of the members preferred that?" "Because all the members prefer it, could we start the work in June?" "Because all the members preferred it, could we start the work in June?" "Could we start the work in June, seeing that all the members prefer that?" "Could we start the work in June, seeing that all the members preferred that?" Your sentences are awkward to read through, and you have to remember that you refer to "the members" and not "members." Also, as far as I know, it doesn't really matter whether you used "prefer" or "preferred" - it depends on the time you are referring to. If the members had expressed, in a previous context (say, three days ago), that they preferred to start in June, then you should probably use the past tense. If however it was expressed recently, say three hours ago, that they would prefer to start in June, then you are probably better off using the present tense. It all depends on context, but in the sentences I gave you above, both can be used. Hope this helped! Cheers :)

Could we start the work in Jun because all members preferred this.?