Learn Dutch: Hoeveel kost die rode strik? - How much does that red bow cost?

You only add the -s when the adjective stands on its own as a noun after words like iets, niets, wat, or veel (for example, iets roods means "something red"). In this sentence, rode is just a normal adjective describing a specific noun (die rode strik), so it follows the regular adjective rules instead.

Why is the adjective rode instead of rood?

Because strik is a de-word (de strik). When an adjective comes before a de-word and follows words like de, die (that), or deze (this), it gets an -e at the end. Notice the spelling change too: rood drops one o to become rode so the vowel sound stays long.

Why do we use die for "that" instead of dat?

You choose between die and dat based on the noun. Because strik is a de-word, you must use die. If it were a het-word, you would use dat (for example, dat rode shirt).