"The only pocket"
Translation:La seule poche
Words that end in "-ment" are usually the adverb form of the adjective they are modifying. It is like adding "-ly" to an adjective in English (bad -> badly, bright -> brightly, soft-> softly...)
Now in English "only" is both an adjective and an adverb, but in French, "seulement" is the adverb of "only" whereas "seule" is the adjective "only". This sentence is modifying a noun, so you need an adjective.
The rule is subjective/figurative adjectives go in front of the noun they modify. Objective/literal adjectives go after the noun they modify.
To simplify the classification a convention/mnemonic device has been developed.
B = beauty
A = age
G = goodness (or badness)
S = size (except when using grand/e when referring to people)
Some comments on various threads have indicated that Number should be included making it B.A.N.G.S.
At any rate, anything that falls into the B.A.G.S. category is very likely to be subjective/figurative and therefore should be placed in front of the noun.
There will be situations where an adjective can easily fall into either subjective or objective category depending on context.
Under the Age classification an old friend can be old in a subjective sense because you have known him for a long time compared to your other friends which places old in front of the noun. Or he can be old in a objective sense because he has lived for a long time. placing the adjective after the noun.
In English you need context to tell which meaning is intended. In French, you know which meaning is intended because of it's position. Just remember this. Changing the adjective's position changes it's meaning.
Because of this Duo will sometimes place a particular adjective in front on some questions and after on others. Because of the short fragments of speech that Duo uses there will be no context to guide you and thus it will seem quite arbitrary as to when they do this. When there is doubt about an adjective's status and there is no context Duo usually accepts either position. (But not always).
Life can seem hard sometimes especially when learning French.