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Looks like rain tomorrow, but let's do our best.
Literal
Tomorrow rain [appears-ようだ] [but-が] let's-do-our-best.
Compact and conversational. ~ようだ ('it seems / it appears') expresses a guess based on available evidence (weather forecast, sky conditions). が is the contrastive conjunction ('but'). がんばろう is the volitional form of がんばる — 'let's persevere / let's do our best.' The contrast between bad weather and determination makes it a motivational mini-statement. No subject is stated — context supplies it.