"The shirt is under the pants."
Translation:Das Hemd ist unter der Hose.
"der" -> Dative feminine, after "unter". "Unter" is a changeable preposition that can take either the accusative or dative case, depending on the situation. While it gets more difficult with idiomatic phrases, an easy way to remember for situations like the one above is "DRAM" - dative rest, accusative motion.