Unique vs. seul
While both unique and seul can be translated as only in English, they are not interchangeable. Unique emphasizes the quality of being one-of-a-kind, whereas seul typically refers to being alone or is used to express a restriction on quantity.
Unique
B1The word unique means one-of-a-kind, sole, or the only one that exists. It emphasizes that something or someone is special, without equivalent, or singular in its category. It never means alone in the sense of lacking company.
C'est mon fils unique.
(He is my only son.)
Cette œuvre d'art est unique au monde.
(This work of art is unique in the world.)
L'unique solution est de partir.
(The only solution is to leave.)
Chaque flocon de neige est unique.
(Every snowflake is unique.)
C'est une occasion unique de le rencontrer.
(It is a unique opportunity to meet him.)
Son unique but dans la vie était d'être heureux.
(His sole purpose in life was to be happy.)
Seul
A2The word seul has two primary meanings, often determined by its position. When placed after a noun or verb, it usually means alone, lonely, or by oneself. When placed before a noun, it means only or single, serving to restrict the quantity of that noun.
Il vit seul dans une grande maison.
(He lives alone in a big house.)
Elle s'est sentie très seule après son départ.
(She felt very lonely after he left.)
J'ai fait tout le travail seul.
(I did all the work by myself.)
Les enfants ne peuvent pas rester seuls.
(The children cannot stay alone.)
Seul Paul connaît la vérité.
(Only Paul knows the truth.)
C'est le seul problème que nous avons.
(It is the only problem that we have.)
Une seule personne a répondu à l'invitation.
(Only one person responded to the invitation.)
La seule chose que je veux, c'est du silence.
(The only thing I want is silence.)
Summary
In essence, use unique to describe something that is one-of-a-kind or special. Use seul to describe the state of being alone (usually after the noun) or to restrict something to mean only (usually before the noun). While la seule chose and 'l'unique chose' can both mean the only thing, unique adds a stronger emphasis on incomparability. Most importantly, you can feel seul (lonely), but you cannot feel unique with the same meaning.