поклясться bedeutet auf Englisch

pledge [pledged, pledging, pledges]▼(to make a solemn promise)
verb
[UK: pledʒ] [US: ˈpledʒ]

swear [swore, sworn, swearing, swears]▼(to take an oath)
verb
[UK: sweə(r)] [US: ˈswer]

vow [vowed, vowing, vows]▼(to make a vow)
verb
[UK: vaʊ] [US: ˈvaʊ]