Learn Hardcore Spanish (Latin America): Mi amigo es único.

Ser is used to describe inherent, permanent characteristics. In this case, being unique is seen as an intrinsic quality of your friend, while estar is reserved for temporary or changeable states.

How does adjective placement influence the meaning in sentences like this? For instance, what difference would it make if you said "Mi único amigo" instead?

In Spanish, the position of the adjective can shift the nuance. When único comes after the noun—as in "Mi amigo es único"—it highlights a distinctive quality. Placing it before the noun, as in "Mi único amigo," could imply that he is your only friend, stressing exclusivity or singularity in count rather than uniqueness in character.

Why does the word único carry an accent mark on the "u"?

The accent mark ensures correct pronunciation. Without it, Spanish accentuation rules would place the stress on the wrong syllable (falling naturally on the penultimate syllable since the word ends in a vowel). The accent indicates that the correct stress is on the first syllable, making it pronounced Ú-ni-co rather than u-NI-co.

Does the adjective change based on the gender of the noun it describes?