I'm trying to find the negation of the sentence "All domestic cars are good". The sentence can be rewritten as "If a car is domestic, it is good".
if P then Q is the boolean expression - P'+Q. The negation of P'+Q is PQ', but I can't make a sentence from that expression. So I'm trying to convert it back into "If.. then.. " form to make a sentence.
Hence the question. If you think PQ' can be written as a sentence, I will consider that as an answer as well.
You're forgetting the quantifiers: if $P(x)$ is '$x$ is domestic', and $Q(x)$ is '$x$ is good', the rewritten original sentence is $\forall x\big(P(x)\to Q(x)\big)$. This can be rewritten as $\forall x\big(\neg P(x)\lor Q(x)\big)$. The negation of this is $\neg\forall x\big(P(x)\to Q(x)\big)$, which is equivalent to
$$\exists x\Big(\neg\big(\neg P(x)\lor Q(x)\big)\Big)$$
and hence to
$$\exists x\big(P(x)\land \neg Q(x)\big)\;.$$
In words, assuming that the universe of discourse is the set of cars, so that there's no need to specify formally that any entity is a car, this is There is a car that is domestic and not good or, more idiomatically, There is a domestic car that is not good.