Questions & Answers about Puer vestigium canis in luto videt et ridet.
Puer is in the nominative singular, the case normally used for the subject of a sentence in Latin.
So in Puer vestigium canis in luto videt et ridet, puer is the one who sees and laughs.
It also matches the verbs videt and ridet, which are both third person singular: he sees and he laughs.
Because vestigium is in the accusative singular, which is the case commonly used for the direct object.
The verb videt means sees, so the question is: sees what?
Answer: vestigium.
A learner may also notice that vestigium is a neuter second-declension noun, and for many neuter nouns, the nominative and accusative singular look the same. In this sentence, its role is clear from the meaning and structure: it is the object of videt.
Here canis is being understood as a genitive singular, so it means of the dog.
That is why vestigium canis means something like .