The probability of an unambiguous sentence is simply the probability of the single parse tree for that sentence.

$P(T,S) = P(T)$

Carleton College

PCFGs are useful in language modeling as they can predict the probability of a sentence or part of a sentence

PCFGs for Language Modeling

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

The probability of an ambiguous sentence is the sum of the probabilities of all the parse trees for the sentence

$\sum_{T s.t. S=yield(T)}^{} P(T)$

 $P(w_i | w_1, w_2, ..., w_{i-1}) = \frac{P(w_1, w_2, ..., w_{i-1}, w_i)}{P(w_1, w_2, ..., w_{i-1})}$