Learn Icelandic: Kötturinn steig á glerið. - The cat stepped on the glass.

steig is the simple past tense of the strong verb stíga (to step). Strong verbs don't use standard past tense endings (like -di or -aði). Instead, they change their main vowel. For stíga, the vowel í changes to ei in the singular past tense. Since the subject is third-person singular (the cat), there is no extra ending added.

Why is "glerið" in the accusative case here? Doesn't the preposition "á" sometimes take the dative?

Yes, á can take either case. It takes the accusative case here because the action involves motion towards a destination: the cat stepped onto the glass. If the cat was already standing still on the glass (showing location rather than motion), we would use the dative case, which would be á glerinu.

What are the parts that make up the word "Kötturinn"?

Kötturinn means "the cat". It is built from the masculine noun köttur (cat) and the attached definite article -inn (the). It is in the nominative case because the cat is the subject performing the action.

How does the word "gler" become "glerið"?

gler (glass) is a neuter noun. To make it mean "the glass" in the accusative case, we attach the neuter singular definite article -ið, resulting in glerið.