Learn Icelandic: Þetta borð er ekki þungt.

Þetta is the neuter form of the demonstrative pronoun (the word for "this"). It is used here because borð (table) is a neuter noun. If the noun was masculine or feminine, you would use þessi.

Why doesn't borð have the definite article attached to it (like borðið)?

When you put a pointing word like þetta (this) directly in front of a noun, you generally just use the plain, indefinite form of the noun. It works just like English: we say "this table", not "this the table".

Why does the adjective end in a "t" (þungt)?

Adjectives in Icelandic change their endings to match the gender of the noun they describe. Since borð is a neuter noun, the base dictionary form þungur (heavy) drops its masculine ending and changes to its neuter form, þungt.

Does ekki (not) always come after the verb?

In simple sentences like this one, yes! Icelandic is a "verb-second" language, meaning the main verb comes in the second position. The negative word ekki usually comes right after the verb, just like in the English "is not".