In German, most verbs in the Perfekt (present perfect) tense form their past participle with either haben or sein as an auxiliary verb. Generally, verbs that express motion from one place to another take sein, while actions that do not imply a change of location take haben. However, some verbs can be used with either auxiliary, depending on their meaning and the way they are used in a sentence.
A common example is fahren. When it describes a movement from one place to another, it takes sein: Er ist nach Berlin gefahren (He traveled to Berlin). Here, the focus is on the fact that the person changed location. But if the emphasis is on the activity itself rather than the movement, haben is used: Er hat zwei Stunden im Stadtzentrum gefahren (He drove for two hours in the city center). Since there is no mention of moving from one place to another, it is treated as an activity rather than a transition. Additionally, if fahren is used transitively, meaning it has a direct object, it takes haben. In Mein Vater hat mich zur Schule gefahren (My father drove me to school), haben is required because mich is a direct object. The father himself is not the one undergoing the movement; instead, he is actively transporting someone else.
A similar pattern appears with laufen. When it means moving from one place to another, sein is required: Sie ist schnell nach Hause gelaufen (She ran home quickly). However, if the focus is on the action rather than the change of place, haben is used: Sie hat zwei Stunden auf dem Laufband gelaufen (She ran for two hours on the treadmill). Same as with fahren, if the sentence includes a direct object, haben is required: Er hat den Marathon gelaufen (He ran the marathon).
With fliegen, the same distinction applies. When the verb describes traveling, it takes sein: Wir sind nach Paris geflogen (We flew to Paris). However, when the emphasis is on piloting or performing the action rather than reaching a destination, haben is used: Der Pilot hat das Flugzeug geflogen (The pilot flew the plane). Here, the focus is not on the fact that he moved from one place to another but on the action of flying the aircraft.
Other verbs that follow this pattern include reiten and schwimmen. Sie ist über das Feld geritten (She rode across the field) highlights movement and requires sein, while Sie hat den ganzen Tag geritten (She rode all day) focuses on the activity and takes haben. Similarly, Er ist durch den Fluss geschwommen (He swam across the river) describes motion and takes sein, whereas Er hat zwei Stunden im See geschwommen (He swam in the lake for two hours) emphasizes the action itself and therefore uses haben.
The choice between haben and sein as auxiliary verbs in the Perfekt tense depends on whether the verb is transitive or intransitive and whether it expresses movement. If the verb has a direct object, it is transitive and always takes haben. When the verb is intransitive, the key factor is whether it describes movement from one place to another—if so, it takes sein. If the focus is instead on the action itself, such as its duration or nature, haben is used. A helpful way to determine the correct auxiliary is to ask whether the sentence answers "Where to?" (indicating movement and requiring sein) or "For how long?" (suggesting an ongoing activity and requiring haben).