Why "ein blaues Hemd" and not "ein blau Hemd" [duplicate]

Question

My query is: Why is "ein blaues Hemd" correct and not "ein blau Hemd"? Could you please give a detailed explanation.

Thanks

Answer 1

The adjective is inflected in three ways, depending on the relationship between adjective and article and/or noun:

1. Ein blaues Hemd liegt im Schrank. (A blue shirt lies in the closet)

Here you have an indefinite article with a so called "weak flection* (ein = a) which means, so to say, the adjective has to do the significant flection work, therefore the flection has to be strong.

2. Das / Dieses blaue Hemd gefällt mir nicht. (The/this blue shirt does not appeal to me)

Here you have a definite article (das = the) or a demonstrative pronoun (dieses = this) which both already have a strong flection, therefore there is no need for the adjective to be significantly inflected and its flection is weak.

3. Das Hemd gefällt mir nicht, weil das Hemd blau ist. (I don't like the shirt because it is blue)

Here there is no need to signify that the adjective is related to the noun by inflecting it because this is the statement of the sentence itself, and the "signifying work" is done by the verb. The adjective is equated with the noun as one of its qualities (Hemd = blau).

After your comment, let us check this with the other genders, as well:

Ein blauer Gürtel liegt im Schrank.

Dieser blaue Gürtel gefällt mir nicht.

Der Gürtel gefällt mir nicht, weil er blau ist.

Thus, no change with male gender (in English, same sentences as above, with a belt – male gender in German – instead of a shirt).

Eine blaue Hose liegt im Schrank.

Diese blaue Hose gefällt mir nicht.

Die Hose gefällt mir nicht, weil sie blau ist.

(In English, same sentences as above, with trousers – female gender in German – instead of a shirt.) So with female gender, the adjective carries no strong flection after the indefinite article. But if you understand the principle, you can easily grasp the exceptions from a grammar table, and indeed, to go into this more deeply you might consider the Schäferhund answers which I think are very good.