So it seems like Cornish is similar to Irish and German in the sense that emphasis of a word is indicated by moving it to the beginning of a sentence. I am guessing that in the present tense, these verb particles "a" and "y" act like direct and indirect relative clause particles in Irish, but I am not sure. It also says that when the subject precedes the verb in a simple affirmative sentence that the verb form must be in the 3rd person singular form. That is, it doesn't agree in both person and number with the subject. Anyhow, this grammar book has the following example sentences:
1a. My a wel an chi. = I see the house.
1b. I a wel an mor. = THEY see the sea.
2a. AN CHI a welav vy. = I see THE HOUSE.
2b. AN MOR a welons i. = They see THE SEA.
3a. OMMA y hwelav an chi. = HERE I see the house.
3b. ENA y hwelons i an mor. = THERE they see the sea.
I wonder if anyone can tell me if these are correct:
4a. My a wel an tren. = I see the train.
4b. Ty a wel an tren. = YOU see the train.
4c. Ev a wel an tren. = HE sees the train.
4d. Hi a wel an tren. = SHE sees the train.
4e. Ni a wel an tren. = We see the train.
4f. Hwi a wel an tren. = YOU (GUYS) see the train.
4g. I a wel an tren. = THEY see the train.
5a. An tren a welav vy. = I see THE TRAIN.
5b. An den a welydh jy. = You see THE MAN.
5c. An venyn deg a wel ev. = He sees THE PRETTY WOMAN.
5d. Chi bras a wel hi. = She sees A BIG HOUSE.
5e. An hen borth a welyn ni. = We see THE ANCIENT HARBOUR.
5f. An den koth a welowgh hwi. = You (guys) see THE OLD MAN.
5g. Ki an tiek a welons i. = They see THE FARMER'S DOG.
Moving parts of the sentence to the beginning for emphasis (fronting) and then making the rest of the sentence into a relative clause is a typical feature of the Celtic languages. I don't speak Cornish, but I suspect your guess about direct and indirect relative particles is probably a good bet.
I think the rule about verb agreement only applies to the third person. It's another typical feature of the Celtic languages that the 3rd person plural form of the verb is only used when directly followed by the 3rd person plural pronoun, otherwise the 3rd singular form of the verb is used; all the Celtic languages do the same thing here (and the rule has even passed into the English dialects of the north of England and Scotland and Ireland).
Thanks for you comment, Coligno. I think you are right about the verb agreement. I read a bit about this passing into some dialects of English, as you stated, too over the weekend. It's pretty interesting. :)