Learn Icelandic: Þetta er stór bíll.

Adjectives in Icelandic must match the gender of the noun they describe. Because bíll (car) is a masculine noun, the adjective takes the masculine singular form, which is stór.

Since bíll is masculine, shouldn't 'This' be the masculine þessi instead of the neuter þetta?

This is a special rule! When you introduce something using the pattern 'This is a...', you always use the neuter word þetta as a dummy subject, no matter the gender of the noun. You only use the masculine þessi when 'this' attaches directly to the noun, like in þessi bíll (this car).

Why does this sentence use the 'strong' declension for the adjective?

You use the strong form of an adjective when it describes an indefinite noun (a noun without 'the' attached to the end). Since we are talking about 'a car' (bíll) and not 'the car' (bíllinn), we use the strong declension.

Does the adjective always go right before the noun?

Yes, usually! When an adjective describes a noun directly in Icelandic, it comes right before the noun, exactly as it does in English (stór bíll = big car).