There are a number of ways to split a multi-syllabic word. It really depends on your approach; there are no hard-and-fast rules. Here are a number of ways to go about it.
The ‘how we say it’ approach
One approach is splitting a word according to how we say it. Take the word ‘wagon’. When we say the word, we say ‘wa-gon’. When readers split a word this way, they are likely to recognise it. This is a linguistic approach to splitting syllables.
The ‘open/closed syllable’ approach
Some teachers teach pupils ‘closed syllables’ that have short vowels in them and ‘open syllables’ that have long vowels in them. They would split the word ‘wagon’ as ‘wag-on’.
The ‘root word and morphemes’ approach
It’s productive to teach children about root words and the grammatical functions of prefixes and suffixes (e.g. ‘re–’ and ‘–ing’). This helps children with comprehension and spelling. In the ‘root word and morphemes’ approach, the word ‘looking’ would be split ‘look-ing’. This is more of a word-meaning route to splitting syllables.
The ‘spelling rules’ approach
Whilst the ‘root word and morphemes’ approach may split running as ‘runn-ing’, some teachers choose to split ‘running’ as ‘run-ning’. This is a rules-based approach, as the pupil will also learn the ‘doubling’ rule: double the ‘n’ when you add ‘–ing’.
Generally, we feel it doesn’t really matter how readers split up words, as long as they can decode the words and derive meaning from them. The only conflict arises with this ‘spelling rules’ approach, when there are double consonants in a word.
The ‘spelling rules’ approach is not compatible with a synthetic-phonics approach. In the word ‘running’, the ‘nn’ is seen as a consonant digraph for the phoneme /n/, and spells just one sound. As we do not split other consonant digraphs, like ‘ck’ or ‘ch’, we should not split the ‘nn’ into ‘run-ning’. As one of the basic principles in synthetic phonics is that two letters can represent one sound, we feel one should be consistent and stick to that principle.