English is not a consistent language. Weird rules and odd exceptions abound. The relevant one to remember a week before the election? The past tense of ‘cast’ is ‘cast’, not ‘casted’.
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The majority of English verbs form their past tense by adding '-ed', including many words with a very similar spelling to 'cast' (think 'boast' and 'boasted' or 'blasted' and 'blasted'). Cast is an exception. The sentence "I cast my vote" can be interpreted as meaning "I am performing the act of voting right now" or "I have already performed the act of voting". But you can't write "I casted my vote". It's simply wrong.
If you want to be absolutely clear, use a form such as 'I have cast my vote'. If you want to be relaxed and casual, say 'I voted' and be done with it.
'Cast' is a complex word; the Macquarie Dictionary lists 62 different meanings for it. You don't necessarily need to learn all of those, but you do need to know that 'casted' isn't right.
The same general principle applies to other verbs which have '-cast' as a suffix, such as 'broadcast' and 'forecast', though you will encounter the -ed versions of these sometimes and the Macquarie allows them. I would not be surprised if in 50 years the exception has disappeared, but it hasn't disappeared yet so you need to be aware of it, especially in formal and professional contexts.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.