This verb form is sometimes left dangling
In many cases, 
such as spell (spelt vs. spelled), learn (learnt vs. learned), and spill (spilt vs. 
spilled), American... [DOC] [TLE] Concise ESL Support - Index - Athabasca University [PAR] Tense Changes from Direct to Indirect Speech; No Change from Direct to Indirect 
... A sentence must have a subject and a verb and be a complete thought.