The English writing system is logical and predictable.
Many of the children I have worked with have difficulty spelling “said.” They sound it out, as they were told to do, and spell it as *<sed>. If I only had a dime!
So let’s go through the four questions.
Said is the past tense and past participle of say.
It has no plus signs, thus <said>
It is related to say.
said
s.ai.d
/sɛd/
The default for /ɛ/ is <e>. So why did this word get an <ai>?
It got an <ai> to mark that it’s related to say!
Say is spelled with the <ay> because no complete native English word ends in <i>.) Said gets an <ai> because English avoids a <y> in the middle of a word (unless to avoid *<ii>, in a compound, or in a medial position of a Greek element).
In English, if you can’t use a <y>, you can use an <i>. And vice versa.
Use a graphic like this to help your students ease in to this perfectly sensible spelling of said. Graphic below.
say
said

Think about it. You just taught your student a lot more about English spelling than how to spell said. Now that’s saying something!

