Rules for messing up the rules

Yey, all is not lost. It is Ok to mess up the strict Iambic Pentameter, by adding a tum-ti here, or an extra syllable here. Whatever I want, it’s cool.
Or is it?
Just when I think I can do anything, I find out there are a load of rules that go with the messing. So here we go:
The first thing I can do is add an extra syllable at the end of a line, to make a weak, or feminine ending. Stephen urges me not to be offended by this title, but to be honest I like to hold on to the notion of being a bit weak and weedy, so I don’t have to carry heavy things.
We then inspect the oh so famous line,
‘To be or not to be that is the question,’
I can’t believe it: it’s not even in strict I.P! I am shocked to my core at the extra syllable on the end there. It then transpires that there are countless instances in which an extra unstressed syllable will be found on the end of a line. For example, when the word ends with ‘ing’ or ‘er’. And the most part of Italian I.P too, as nearly all of their words end with ‘a’!
It would seem natural to conclude from this that it is also alright to take a syllable off if you want to. But it is most certainly not! It is totally unthinkable, terrible and ridiculous as it would leave the line with a disastrous four stressed syllables instead of five. So don’t do it!
The next thing is substitutions. Firstly the trochaic substitution, or tum-ti instead of ti-tum. This means simply the first syllable is stressed instead of the second, and is most commonly found at the beginning of a line. The second is spondaic substitution, which is two stressed syllables together. I’m looking at the examples I’ve been given and thinking that really if it wasn’t pointed out, then I wouldn’t notice these changes at all, but hey I’ll keep trying.
The final type of substitution is pyrrhic. This is where there are two unstressed syllables together, the second one having been ‘demoted’ from stressed status. Sorry dude, I know this was your one big chance to shine, but we’ve had to put you with the unstressed syllables. Move along now.
Pyrrhic substitutions, because they seem so down-and-out, are definitely my favourite so far. And hooray, there is a glimmer of hope for them, as they help to further emphasise the next stressed syllable that follows them. How selfless they are, sacrificing themselves for another’s glory!
Anyway, that’s about it. It’s nice to know I can mess around with the beat of I.P, rather than trying to squeeze syllables in where they don’t sound good. I’m hoping I can use this new knowledge to my advantage...