What is the grammatical difference between beau and bel? They are both the masculine singular form of the adjective beau, right?
"Beau" and "bel" are both french adjectives of the some token. That is that 'beau' becomes 'bel' before a french singular masculine noun starting with a vowel or a mute 'h'. Hence, if we take the nouns "Art" and "homme", the plural of beau stays unchanged while becoming plural: "les 'beaux' arts" or "l'Ecole des 'Beaux' Arts de Paris".
So, 'beau' keeps its form when qualifying a plural noun here, 'arts'.
But if art was singular, it's 'bel' that would has been used.
Example: "Quel 'bel' art !" or "Quel 'bel' homme !" and "Quels 'beaux' hommes !" for the plural.
While using the same process we would have said: "Quelle 'belle' femme !" for the singular, and "Quelles 'belles' femmes !" for the plural.
We will here use the same process for:
"Quel 'vieil' homme !" and "Quels 'vieux' hommes!".
Where, 'H' is also mute.
Actually, it is pretty simple. If there is a vowel after "beau", it becomes "bel":
Il est bel et bien avocat.
Quel bel écureuil !
Quel beau cheval !
You can see more exemples and exercises here.
Yes, they are both the masculine form of beau, but bel is used when the noun after it starts with a vowel. For example,
Un bel appartement
Note that bel is before the noun. Follow the rule called BANGS word. B stands for Beauty, A stands for age, N stands for new/old, G stands for good, and S stands for size. If any adjective falls in a category of BANGS, then it will go before the noun. Usually they go after the noun.