Learn Farsi: اگر مجرم دروغ بگوید، قاضی محکومش می‌کند. - If the criminal lies, the judge will convict him.

Because محکوم کردن (to convict) is a compound verb, made of a non-verbal word (محکوم, meaning convicted) and a light verb (کردن, to do/make). When using an object enclitic like ـَش with a compound verb, it is most natural in Farsi to attach it to the end of the non-verbal part. So we say محکومش می‌کند rather than attaching it to the very end of the verb.

Could I use او را instead of the attached ـَش in this sentence?

Yes, absolutely! You could say قاضی او را محکوم می‌کند (the judge convicts him) instead of قاضی محکومش می‌کند. Using the enclitic ـَش is just a very common, slightly more conversational alternative to using an independent pronoun with the object marker را.

Why is the verb بگوید in the subjunctive form here?

In Farsi, the 'if' clause of a conditional sentence almost always takes the present subjunctive because the action is hypothetical or uncertain. That is why we use the subjunctive بگوید (from گفتن, to say/tell) right after اگر (if).

The English translation says 'will convict', but isn't می‌کند in the present tense?

Exactly! In real conditional sentences like this one ('If X happens, Y will happen'), Farsi typically uses the simple present tense in the main clause, even though English switches to the future tense ('will convict'). The present tense here easily implies a future result.