What is blank verse? Iambic pentameter? Can you give me some examples from Othello?

I don't know that I'm the one to try to help you with a
breakthrough in your understanding of rhythm and metre.  This can be very difficult and
it seems to be something that students learn or "get a feel for" over time.  It can,
also, be a little bit subjective on the part of the poet.  Sometimes stress is
determined by the context, and a one-syllable word that is stressed in one sentence is
not stressed in another.  Also, no serious poet repeats the same rhythm every line--it
becomes monotonous.  So a student looks for the rhythm that is dominant in the pattern
of the lines, not exclusive.

With all that said, here's an
explanation that I hope will help.

When you speak in the
English language, you speak in a natural rhythm.  Your voice rises and falls with almost
every syllable.  This is inflection.  You cannot, for the most part, speak sentences in
English and speak at the same inflection.  Your voice naturally rises and
falls. 

Look at the word:  hippopotamus.  On which syllable
does your voice rise?  You must get yourself to say the word naturally, like you would
any other time. 

When you speak the word in English, your
voice naturally rises on the middle or third syllable, the -po-.  That is the
stress:  hippopotamus.  That's all there is to stressed and
unstressed syllables.  And any word in the English language that is two or more
syllables, is marked for stressed and unstressed syllables in the dictionary.  One place
to start in trying to understand rhythm is to simply look up any words with two or more
syllables in the dictionary and see where the stresses lie.  They will be marked with a
symbol before the syllable that is stressed, where the dictionary gives pronunciation. 
 

The same is true when a person speaks a sentence.  Your
inflection naturally rises and falls as you speak a sentence, the same as it does when
you speak a multi-syllabic word.  You speak the sentence as you would at any other time,
and judge when your inflection rises and when it
falls.  

Here's a short line from a famous Frost poem, with
the stresses in bold:

Her
hardest hue to
hold       

This line is
iambic tetrameter:  three metrical feet of iambs. 

Here's a
line
from Macbeth:

Attend
the true event, and
put we
on

If you look up
attend and event in the dictionary, and learn
where the stresses are, that will give you a great start in figuring out where the
stresses are in the entire line. 

I hope this helps.  By
the way, you probably figured this out already, but just to make sure you don't leave
this site with a misconception, the Roses are Red poem mentioned above is not iambic
pentameter, and all of the lines mentioned are not iambic.  Also, a listing of letters
like ABAB is used for rhyme scheme, not to indicate rhythm and
metre.