Grammar girl here, a listener named Rob tweeted me a question about contractions.

He asked, why is it okay to say you're correct or you're welcome, but not I'm smarter

than you're?

This isn't so much a question about grammar as about English phonology.

It reminds me of a very short poem on my red one time, which seems to have been written

by someone named Ethel Barrett DeVito in the mid-20th century, though I haven't been

able to track it down fully.

It seems to be a complaint about either highways or checkout lines, and it goes like this,

wherever the place, whatever the time, every lane moves, but the one where I'm.

The poem is funny at least in my opinion, not only because it's so true, but also because

in order to make a rhyme, the author has deliberately used the contraction I'm, which just

doesn't sound right in this situation.

But why doesn't it?

The Cambridge grammar of the English language describes the situation like this.

Some English words, in particular function words, such as pronouns, prepositions, and

auxiliary verbs, have both strong and weak forms.

For example, the strong form of the pronoun you is you, the way you pronounce it when

you're reading it all by itself.

However, it also has a weak form, which you'll hear in sentences like what do you want

for lunch.

Did you hear how it was pronounced to ya?

Sometimes this weak form is even spelled YA.

...