Background. Written composition requires handwriting, spelling, and text-planning skills, all largely learned through school instruction. The rate at which students learn to compose text in their first months at school will depend, in part, on their literacy-related abilities at school start. These effects have not previously been explored.Aim. We aimed to establish the effects of various literacy-related abilities on the learning trajectory of first-grade students as they are taught to write.Sample. 179 Spanish first-grade students (94 female, mean age 6.1 years) writing 3515 textsMethod. Students were assessed at start-of-school for spelling, transcription fluency, letter knowledge, phonological awareness, handwriting accuracy, word reading, and non-verbal reasoning. They were then taught under a curriculum that included researcher-designed instruction in handwriting, spelling, and narrative planning. Students’ composition performance was probed at very regular intervals over the first 13 weeks of instruction.Results. Controlling for age, overall performance was predicted by spelling, transcription fluency, handwriting accuracy, word reading, and non-verbal reasoning. Most students showed rapid initial improvement, but then more gradual learning. Weak spellers showed weaker initial performance, but then improved steadily across the study period.Conclusion. Findings suggest the need to assess writing as a learned skill that develops differentially across students in response to specific instruction, rather than as a static ability.