ぼくたちはいつも規則を守らなくてはいけない。

we

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

always; usually; every time; never

rules; regulations; conventions

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to protect; to guard; to defend; to keep (i.e. a promise); to abide (by the rules); to observe; to obey; to follow

if ... then; when; and; why don't you...?; the more (one does something); because; since