per-tú-
“crossing, passage, ford”port, portal, transport, passport
Abstract of *per- giving Latin portus, English port, portal, transport, passport, export, import.
Discussion
The PIE form *per-tú- (a crossing, a passage, a ford) is a result noun from *per- (to go through, to pass, to cross), formed with the *-tú- suffix that creates abstract nouns of action. The meaning is "that which results from crossing" — a ford, a passage, a point of transit.
Latin portus (harbour — the crossing-point where ships pass from sea to land) continues the root and gave English: port (a harbour), portal (an entrance — a crossing-point between spaces), transport (to carry across), export (to carry out), import (to carry in), deport (to carry away), passport (a document permitting passage through a port), opportunity (ob-portūnitās, "that which lies before the port" — the favourable moment when the harbour is accessible), and the combining form -port in all transit vocabulary.
English ford (OE ford, from PGmc *furduz, from *per-tú-) continues the root natively — a ford is a crossing-place in a river, the point where water can be traversed on foot. Oxford ("oxen-ford"), Stanford, Bedford, and hundreds of other English place names preserve the PIE crossing-word in their geography.
German Furt (ford) and the related Fahrt (journey, drive) confirm the Germanic cognates. Frankfurt ("Franks' ford") names the city by its river crossing.
The root connects to *terh₁- (to cross over — see the trans/through root) as part of the PIE vocabulary of transit. Where *terh₁- emphasised the act of crossing, *per-tú- named the place where crossing occurs.