Changes to the Stem
Spanish: Edexcel IGCSE, topic 10.3.2
Changes to the Stem
When a verb has an irregular stem in the 'él/ella' form in the preterite, the gerund follows the same irregular pattern (e→i and o→u). For example:

Pedir → pidiendo

Venir → viniendo

Dormir → durmiendo

Preferir → prefiriendo
Test yourself on Changes to the Stem
How do you say 'sleeping' in Spanish?