In the Early years children are taught the 44 phonemes in the English language that they will need to use in their writing. The focus is more on learning to read than learning to write at the start of the year but they very quickly learn to write confidently.
The children in Key Stages 1 and 2 will have regular spelling tests to help them with words they can sound out and also topic words. It is really important that children learn to spell phonetically (by sounding out) but that they also learn to spell over the 2 key stages the most commonly used words which can't be sounded out so easily (high frequency words).
Open the PDF's to see which words your children will need to spell by the time they leave Year 4
There is also a link to a page that tells you all the sounds the children need to learn by the end of each year group.