Learn Icelandic: Strákurinn er svangur og hann borðar mikið.

This sentence uses hann (he), which requires the third-person singular form of the verb. The verb borða (to eat) is a Group 1 weak verb. In the present tense, these verbs take an -a ending for I (ég borða) and an -ar ending for you, he, she, and it (þú / hann / hún / það borðar).

Why is the word for hungry spelled svangur here?

Adjectives in Icelandic change their endings to match the gender of the noun they describe. Because Strákurinn (the boy) is a masculine noun, the adjective takes the masculine singular ending, making it svangur. If the subject were a feminine noun, like a girl, it would change to svöng.

I see the English translation says "The boy", but where is the word for "the"?

In Icelandic, "the" usually isn't a separate word like it is in English. Instead, it gets attached to the end of the noun as a suffix. The base word is strákur (a boy). Adding the masculine suffix -inn turns it into Strákurinn (the boy).

How is the word mikið being used here?

It is acting as an adverb to describe the action of eating. While it comes from an adjective meaning "large" or "much", the specific form mikið is very commonly used right after a verb to mean "a lot". Here, it simply tells us the amount that he eats.