Learn Icelandic: Hér eru tvær krukkur og eitt egg.

In Icelandic, the numbers 1, 2, 3, and 4 change to match the gender of the noun they are counting. The word for "jar" (krukka) is feminine. Therefore, you have to use the feminine form for "two", which is tvær.

Why is "one" translated as eitt here instead of einn?

Just like with "two", the number "one" has to match the gender of the noun. The word egg is neuter, so we use the neuter form of the number, which is eitt.

What is the singular form of krukkur (jars), and how can I tell its gender?

The singular form is krukka. In Icelandic, nouns that end in "-a" in their basic dictionary form are almost always feminine. To make them plural, the "-a" usually changes to "-ur".

Why do we use the plural verb eru (are) if the sentence ends with just one egg?

The verb eru matches the whole subject of the sentence, which is "two jars and one egg". Since you are talking about multiple items in total, you use the plural "are" (eru) just like in English.