Intonation and pronunciation: 2
The metal box should not have been left on the floor. I could of hurt myself.
This is a mistake that is being made a lot by native speakers, and it is one of those errors that only shows itself when it is written, and not spoken. It is a problem that might show itself in written reports, for example, such as accidents.
What the speaker is actually saying 'I could've hurt myself.' 'Could've' is the contracted form of 'could have', NOT 'could of'. But when spoken, we say the contracted form, which does sound a lot like 'could of'.
The same problem arises with should: You should of informed the police. (WRONG! You should have [should've] informed the police.) Would: You would of been successful in the negotiation. (WRONG! You would have [would've] been successful in the negotiation. Will: When the deadline arrives, we will of thought of a solution. (WRONG! We will [will've] thought of a solution.
Indeed, this rule follows with virtually all modal verbs used in the perfect tenses - can, may, might, must, shall, have to, got to, ought to, be supposed to, be going to, used to: You must of made a mistake. (WRONG! Must have [must've].) We were supposed to of met them at the airport. (WRONG! supposed to have [supposed to've].)
Although the contracted forms are okay and are used in spoken English, it is recommended that you do not use them in written English, except when used very informally such as emails, messaging and social media. Use the full form instead: You could've hurt yourself. (FORMAL: You could have hurt yourself.)