A. The Three-letter rule

Structure words have less than three letters; content words can be any length, from three letters upwards (but must not have less than three letters):
  so:sew/sow    to:two/too    we:wee    oh:owe           by:bye/buy    no:know   
  an:Ann           I:eye/aye        in:inn      be:bee             or:ore/oar/awe         

B. The TH Rule

In structure words, the initial <th> spelling corresponds to //, ‘this’ and ‘they’; in content words,

initial <th> corresponds to // as in ‘thesis’ and ‘Thelma’.

the:therapy  than:thank  thou:thousand   this:thistle   thy:thigh   though:thought
that:thatch  those:thong them:thematic

C. The Titles Rule

In titles of books, films etc, content words usually start with capital letters, structure words with lower case.

The Case of the Stuttering Bishop   The Handbook of Bilingualism    Strangers on a Train

The Tragedy of King Richard the Second    I Wish I could Shimmy like my Sister Kate