(Name of the other side)  Date: 
(Address of the other side)
Dear Sirs,
Re : (the Property address)
Landlord : (your name)

I/We* am/are* writing to inform you that you have breached the clause X (please insert the relevant number of the clause in the tenancy agreement) of the Tenancy Agreement signed by me/us* and you on (please fill in the date of the Tenancy Agreement) ("the Agreement"). (you can also list out the detail of the breach, for example, to sub-lease the properties to third parties, to store some dangerous goods inside the properties)

Despite repeated demands, you have failed to response to my/our* warning and let the said wrongful acts to continue. (you can deleted this sentense if it is the first time you warn the other side on the his wrongful acts)
I/We* therefore give you notice pursuant to Section 58(1) of the Conveyancing and Property Ordinance that you are required to remedy such breach of the Agreement within the next 7 days otherwise legal proceedings will be instituted against you without further notice in which event you may be liable for the legal costs occasioned thereby.
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Yours faithfully,

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(signature here)

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(Block Letter of you name)

* Please choose one