The grammar of modern languages, including English, began to be written in the Renaissance, beginning with Spanish. Major revisions took place in the twentieth century, as part of the new academic disciplines of linguistics and applied linguistics. None of these movements took the developing language of the child as their starting point, and all use terminology that is more suited to adults, not to say postgraduate linguistics students. The approach set out here began in an FE college with students who were likely to fail because they couldn't write in sentences, but recent work with primary pupils has shown that it can equip younger children with what they most need to know about grammar as they write. I'm giving it a working title of go-to grammar.
1. A sentence begins with a capital letter and ends with a full stop. (!? contain full stops)
2. Nearly every sentence has a verb - sentences without a verb are rare, but there are some, so we say nearly. Most verbs do things, but to be and to have, the most frequent verbs, do not represent action, and some verbs represent feelings. Children need to understand what verbs are - this takes explanation and practice. It is fine to say that most verbs do things, but not that all verbs are doing words.
3. Nearly every sentence has a subject - not topic, but who is doing or feeling or being what the verb is. To find subject, we go to the verb and ask a joke question such as Whodunnit? or Who/what is it? The subject-verb axis is the key to sentence construction in European languages, including English. It takes a little practice to find the subject, but almost all children can learn to do so.
4. Repeat or change subject and you need strong punctuation (anything with a dot or dash in it) or a link word. Key point. Link words include and, but, because, although, and relative pronouns - who, which, that. The term connective means the same thing as link, and there is no point in using a long word when a short one will do.
5. Sometimes you need a starter word rather than a link word. Starter words allow us to put more than one idea into a sentence. Grammarians prefer the term "subordinator", an ugly term that they produce by turning the adjective subordinate into a verb for their own benefit - it says nothing to children at all. It can be introduced later if preferred, but I find it doesn't much help.
6. Finally, sentences are like a meal out. Dogs mix everything up and gobble it down, but humans keep things organised so that tastes don't get mixed up and we don't end up with mustard in our pudding. Main course, fine. Starter + main, fine. Main + dessert, fine. Starter, main + dessert, fine. Add coffee, cheese, aperitif if you like, but not too often or you'll get indigestion.
This is the core, to which finer points can be added as the learner is in a position to understand them.