A sublease happens when a tenant leases to a third party. The third party pays rent to the tenant who, in turn pays rent to the landlord. You end up with two relationships: landlord/tenant and tenant/subtenant. If the subtenant has problems with the space, then he deals with the original tenant.
An assignment happens when the tenant signs away their rights and responsibilities to a third party for the remainder of the original lease. In this case, the third party deals directly with the landlord and the original tenant is pretty much out of the picture. However, if the third party tenant defaults on the lease, then the original tenant remains liable for the remainder of the lease and the landlord can sue the original tenant.
Subleases are typically short term but do not become an assignment without an additional agreement. It seems like you've been a good tenant by keeping your sublandlord out of trouble with the landlord. Check in with the county to see if you live in a rent controlled area. If so, even the sublandlord must comply with the local rules regarding affordable housing.
I suggest talking with a landlord/tenant attorney in your area to explore your rights to see if there are any violations. There are more than a few attorneys out there who could give you a free initial consultation.
This is not a GP question... I'll fix it.
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