Can you give me an example of iambic tetrameter and trimeter?

"Iambic" means the unit of sound is two syllables with the second one stressed.--di DAH di DAH di DA
Each of these two syllable units is called a foot.
Tetrameter means that each line of the poem contains four of these feet.
Lines of iambic trimeter contain three feet (not a yard- three feet).

Example: Iambic tetrameter--
I'm going to the zoo today,
To watch the zebra chew some hay.
Until a lion jumps the wall,
And chews the zebra
Bones and all.

Example: Iambic trimeter--This one is a little harder, and if you string too many lines of iambic trimeter together in a row it can get monotonous. Here is a try--
Come with me now my son,
To see the great divide.
I'll show you where the one,
Called Billy the Kidd did ride .

He robbed trains with his men,
The gun was all he knew,
He shot two score and then,
Was killed at twenty-two.

Hope this helps. It's not Longfellow but it's the best I can do on short notice.