nächsten Ampel

Hallo .

The traffis light is : die Ampel . So then when we say , in the next traffic light , why does it change to " an der nächsten Ampel " . Is it because the preposition 'an ' , changes die Ampel to der Ampel in Dative ? But why 'nächsten' ? 
The sentence is : Hier geradeaus und an der nächsten Ampel links.

Vielen Dank!
Vidya 

As you say der Ampel is dative, so the adjective nächst also needs an ending.
There are 3 sets of adjective endings:
Weak after der die das
mixed after ein
and strong  when there is no article.

I don't think this is covered until level 2.

It is a bit complicated  but there is an explanation here:
https://www.learn-german-smarter.com/learn-german-adjective-endings/
Or you can google german adjective endings for other examples.

 

Hallo Vidya-manuS und sfpugh,

As you both said, the dative case is needed here. The preposition "an" can indicate the dative as well as the accusative case.

In order to find out which case is needed you can ask the question "Wo?" - "Where?" (position or location) for the dative case and "Wohin?" -"Where to?" (movement) for the accusative case. 
The sentence above "an der nächsten Ampel" - "at the next traffic lights" indicates a location, therefore the dative case is used. 

In addition to changing "die Ampel" to "der Ampel in dative, the adjective has to decline as well. This adjective ending depends on the nature of the article, the case, and the gender of the noun. 

The dative case is covered in lesson 9.11 "A Case in Point - Dative" and 14.7 "German Cases – Part 2". You can learn more about adjective endings in lesson 10.8 "Adjective Endings". 

Bis zum nächsten Mal!
Julia