"The architect builds the bathhouse."
Translation:Architectus balneum construit.
"Condere" is "to build" in the sense of "to found/to establish". It's more appropriate for cities than for individual constructions.
They really need to start making context clear during the lesson instead of putting it in the tips (if it is). And making sure it's clear when translated to English that we're referring to The Baths, not 'the bath'. Otherwise we get these nonsensical sentences like "the tub is in the bath" which is completely meaningless even with wider historical context.
This sentence does not use the ablative. But this might help you:
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.