Learn Icelandic: Þú greipst leikfangið, þú ert heppinn.

The dictionary form is grípa (to catch or grab). Because it is a strong verb, it doesn't take a standard past tense ending like -ði. Instead, its core vowel changes from í to ei, creating the past tense stem greip. For the

Why does leikfangið end in -ið here?

Leikfangið means the toy. The base noun is the neuter word leikfang. The verb grípa requires the object being caught to be in the accusative case. Since neuter words look exactly the same in the nominative and accusative cases, we just add the standard neuter definite ending -ið to mean the.

If þú could be anyone, why is heppinn (lucky) in the masculine form?

In Icelandic, when the gender of the person you are speaking to is unknown or general, adjectives default to the masculine form. If you were speaking directly to a woman and knew it, you would use the feminine form heppin (with one n) instead.

What are the root words that make up leikfangið?

It is a compound word made from leikur (a game or play) and fang (a catch, grasp, or item held). So leikfang literally translates to something like play-thing.