Learn French: Le menuisier est certes très fatigué, mais il doit poncer ce bois. - The carpenter is admittedly very tired, but he must sand this wood.

Using just mais simply links two contrasting ideas. Adding certes (admittedly/to be sure) turns it into a structured concession. You are explicitly validating the first point (yes, it is true he is tired) before overriding it with your main argument (he still has to work). It is a classic rhetorical tool in French argumentation.

Why is certes placed directly after the verb est here? Could I start the sentence with it?

Yes, you can absolutely start the sentence with it: Certes, le menuisier est très fatigué, mais... Placing it after the conjugated verb is also very common and sounds slightly more integrated into the flow of the sentence, whereas putting it at the beginning adds a stronger, slightly more formal emphasis to the concession.

Is the final 's' in certes pronounced?

Yes, it is! Unlike many French words ending in 's', the 's' in certes is always pronounced. It sounds like the English word "cert".

I know sable is the noun for "sand", so why is the verb for sanding wood poncer instead of something like "sabler"?

In French, sabler usually means "to sandblast" (cleaning or etching with high-pressure sand) or to scatter sand. For smoothing wood or walls with sandpaper (du papier de verre), the specific verb is always poncer.