Concerning the superlative it is to be distinguished between the use as predicative or adverb on the one hand side and attributive adjective on the other hand side.
Compare
Use
as predicative adjective
Er ist am
stärksten.
= He is the strongest.
Use
as adverb
Er läuft am
schnellsten von allen.
= He runs the fastest of all.
Use
as attributive adjective
Das schönste Gedicht
war das von Rilke.
= The most beautiful poem was by Rilke.
Sie ist die
schönste Frau der ganzen Stadt.
= She is the most beautiful woman of the whole town.
If we have a look with some concentration we will find a difference. When using the superlative as predicative adjective (put behind the object it describes) the preposition am
is used and an 'n' is added to the superlative form.
Er ist am stärksten.
In other words one can say: If there is no article or possessive pronoun before the superlative the construction to be used is am
+ adjective+ -sten.
Am + adjective + -sten
Er ist am
stärksten.
= He is the strongest
but
Er ist der
Stärkste.
= He is the strongest.
In English there is no difference between a predicative adjective and a substantivated adjective. In German it is the use of preposition am or the definite article der/die/das.