Verbs continuing the Rückumlaut pattern

Many verbs derive from Germanic strong verbs, which make many of their inflected forms through vowel gradation, as can be observed in Modern English patterns.

Verb	Past	Past Participle
bring	brought	brought
buy	bought	bought
catch	caught	caught
seek	sought	sought
sell	sold	sold
teach	taught	taught
think	thought	thought

To find the correct form, you can use an online database of the irregular verbs.

For example: irregular verb BUY