infra

Infra takes the accusative and only the accusative, like its opposite supra and its neighbours intra and extra. That is the first thing to fix, because sub, the other word for "under", takes the ablative when it means position and the accusative when it means motion, and learners import that split where it does not belong.

It doubles as an adverb, and that is its commonest job in scholarly Latin: ut infra, "as below", vide infra, "see below", pointing further down the page. The contrast with supra runs through legal and academic citation to this day. Infra also means "less than" with numbers, and "lower in rank", which is where English infrastructure, inferior and infernal come in, all off the same inferus, "low".