"This wall is harder than that wall."
Translation:Deze muur is harder dan die muur.
It has to do with whether a word has the article de or het. De-words are more common, and include all plurals. There are some rules but mostly you just have to learn it.
De-words have die and deze, het words have die and dat. It is 'de muur', so it becomes 'harder dan die muur', or 'harder dan deze muur'.
If you take a het-word, say 'het hout', it becomes 'harder dan dat hout', or 'harder dan dit hout'.