This is why we need to mark long vowels!
can also be expressed as
For this reason, we need to mark long vowels!
In my opinion, “this is why” is lower register English. In archaic English, we would find the word “wherefore.” In Koine Greek, we would find διό. In Latin, propter hoc. (This is not to say that there is only one way of expressing that English phrase in Greek or Latin.)
Consider the following passage in Matt. 27:6–8:
6 The chief priests picked up the coins and said, “It is against the law to put this into the treasury, since it is blood money.” 7 So they decided to use the money to buy the potter’s field as a burial place for foreigners. 8 That is why it has been called the Field of Blood to this day. NIV, ©2011
The Greek text has διὸ and the Vulgate propter hoc. According to Lewis & Short,1
B. Trop., in stating a cause.
1. On account of, by reason of, from, for, because of (syn. ob; “the predom. signif. of the word): parere legibus propter metum,” Cic. Par. 5, 1, 34; cf.: “propter metum poenae,” id. Sest. 46, 99: “propter eam ipsam causam,” id. de Or. 1, 16, 72: “propter frigora frumenta in agris matura non erant,” Caes. B. G. 1, 16: “propter humanitatem,” Cic. Att. 7, 5, 2: “is propter morbum exire non potuit,” Auct. Her. 1, 11, 19: “excusato languore faucium propter quem non adesset,” Suet. Ner. 41; id. Aug. 8; Sall. J. 23, 1: bidentes propter viam facere, to sacrifice on account of a journey, Laber, ap. Non. 53, 26; cf.: “propter viam fit sacrificium quod est proficiscendi gratiā, Herculi aut Sanco,” Fest. p. 229 Müll.; Plaut. Rud. 1, 2, 62: “propter injuriam,” to avoid injury, Pall. 12, 13: “propter hoc, propter quod,” on that account, for which reason, Varr. R. R. 3, 16, 14;
1 p. 1472, propter
Lewis, Charlton T.; Short, Charles. Harper’s Latin Dictionary: A New Latin Dictionary Founded on the Translation of Freund’s Latin-German Lexicon. New York: Harper & Brothers, 1884.