Verbal nouns were known as gerunds in more traditional grammar. The word is more useful in the study of Latin grammar than in the English variety.
Sometimes verbs in the -ing form can be used in a phrase in the same way as nouns. (Usually, as we have seen, the -ing form is used in a way close to an adjective.) "Studying makes me hungry," we say. Subjects should be nouns, or noun phrases. In this example, the Subject of the Verb ('makes') is 'studying'. So here, an -ing form that is usually the present participle is a noun.
In "Crossing the street can be dangerous," the -ing form is also a noun, the Subject of the verb 'can be'. But it is also a verb in its own right, with an Object - 'the street'. We see the same grammatical construction if we adapt the first example to "Studying grammar makes me hungry." This can be shown as
[(Studying (verbal noun) grammar (noun)Od)S makes (verb) me (pn)Od hungry (adj)Ca]
There are many complexities in the uses of verbal nouns. These are interesting to those who study the theory of grammar, but they are too many to cover here.
What is important for people starting to study grammar is to realise that there are such things as words with dual use - that sometimes words have two functions at the same time. For everyday purposes, it is enough to know that there are such things as verbal nouns and participles. They can look alike; they must be distinguished by their function.