Questions & Answers about Er fährt mit dem Zug.
Why does the German sentence use mit instead of a word for "by"?
In English we say we travel "by" a vehicle, but in German you always use mit (which normally means "with"). You are literally saying "He travels with the train."
Why does it say dem Zug instead of der Zug?
The German word for train is masculine: der Zug. However, the preposition mit is a strict dative preposition. This means it always forces the noun following it into the dative case. For masculine nouns, der changes to dem.
Why is there an umlaut on fährt?
The dictionary form of the verb is fahren. It is a strong verb that changes its stem vowel from "a" to "ä" in the present tense, but only for the "you" (du fährst) and "he/she/it" (er/sie/es fährt) forms.
If I want to say "He is going by train," could I use the verb gehen?
No, you cannot use gehen here. In German, gehen strictly means to walk or go on foot. When you are taking a vehicle with wheels like a train, bus, or car, you must use fahren.