Without the tenant knowing? Certainly. Being sued often comes as a surprise.
Without the landlord knowing? ... that would be more difficult, but it's possible that someone in the landlord's office initiated the suit based on standard policy without the landlord being personally aware of the details.
See answer to related question, "Can a tenant sue a landlord for trespass?"
The landlord could sue the decedent's estate.
I can't answer definitely without more information. Basically, if the tenant did something wrong, then the landlord may very well be in a position to sue to be compensated for this wrong.
Yes. The tenant should be considered the landlord of the sub-tenant. Therefore, he can evict, just like any landlord.
In most states, if a lease term is for a fixed amount of time, such as a year, and the tenant breaks lease by moving out early, the landlord can sue for the amount of time it took for the landlord to get a new tenant or for lease to expire, whichever comes first. It is for a month-to-month tenancy, then the landlord has no grounds for suing for future rents.
Not unless the landlord has followed your state's laws regarding disconnection of utilities for nonpayment. Any landlord who disconnects the tenant's utilities (except temporarily for maintenance purposes) without following these laws will be guilty of constructive eviction. A constructively evicted tenant can sue the landlord for moving expenses and damages.
Landlord can keep deposit and sue tenant's estate if damages to the apartment was done outside of normal wear and tear.
If the tenant's health issues are caused by the landlord's violation of health and building codes, then the tenant can sue the landlord. The landlord's insurance would normally get involved at this point, as opposed to being sued directly. If the tenant has special health needs that the landlord isn't meeting, then too bad, move. As long as the health and building codes are being complied with, tenants cannot force a landlord to take extraordinary measures.
Only if it's within the statute of limitations.
It depends. If the sub-tenant was there by right and the landlord allowed a sub-lease, then that's a situation where nothing has gone wrong. If the landlord is suing, it sounds instead like the tenant did not have the right to sublet and in that case they are responsible to complete the lease and the sub-tenant may not have had the right to be there.
In most states if there is evidence of abandonment of the property by the tenant, the landlord can reclaim the property without going to court. If the tenant owes money the landlord can sue in absentia (must use tenant's last known address) for unpaid money and damages if applicable. If he leaves personal belongings you can dispose of them as you see fit.
Yes, you may have grounds to sue your landlord for locking and pawning your belongings without proper legal notice. Landlords have a legal obligation to follow certain procedures when dealing with tenants' belongings. It's advisable to consult with a lawyer who specializes in landlord-tenant law to assess your specific situation and guide you in taking appropriate legal action.