Can a Landlord enter your apartment without letting you know? Can they knock and just let theirself in?

Only in the case of an emergency (such as fire or a leak) or if your lease specifically says so. If your lease does not specifically state that the landlord may enter at any time then the landlord must provide you with reasonable notice which is usually defined as 24 hours.

Unless there is an emergency, the landlord must give you notice of the intent to enter your apartment. With a 24 hour notice, he is permitted to enter the apartment whether you are there or not. Could we have a few more details as to why the landlord entered the apartment? Most landlords will find a reason to enter about 4 times per year. They'll change a furnace filter, batteries in the smoke detector, check for water leaks etc. They will ask to enter the property if they are considering selling and need to show a realtor. Sometimes the insurance agent will want to do a walk through before writing a policy. Each time they walk through they are also checking to be sure you are keeping the property in a safe and orderly manor and not breaking the lease in any way. The rental property is probably one of the landlord's most expensive assets and he needs to protect it.