Section 5-12-130(e) defines abandonment to include giving "actual notice to the landlord by the tenant indicating the tenant's intention not to return to the dwelling unit." Although the responsibility for rent continues, the landlord is under a good-faith duty to re-rent the apartment at a fair rental. If re-rented at or above the tenant's rate, the tenant will not owe money. Until re-rented, the tenant is on the hook for rent for that period. And if re-rented for lower rent rate, then tenant is responsible for the difference between the rates.
The tenant is also be liable for the reasonable advertising expenses and reasonable redecoration costs incurred by the landlord in trying to re-rent the apartment. The plus side is that if the tenant is sued for the unpaid rent, there is a defense (which could mitigate or defeat landlord's claim) if landlord did not act in good faith or make the effort to re-rent.
Offering a sublease
To piggy back on the last option, if the tenant tries to terminate without right, the landlord must accept a reasonable sublease proposed by the tenant without an assessment of additional fees or charges. See Section 5-12-120. If the sublease doesn't cover the full rent owed by tenant, the same rules as in the abandonment section apply.
Look to the contract
Sometimes a lease may provide for an "early exit" procedure to cut off rent for the full lease term. Tenant's should make sure to read the lease carefully.
Talk with your landlord/leverage potential counterclaims
Sometimes the landlord may allow a tenant to leave early with reduced or no penalty. This may be more likely if the reason for early termination compels the landlord to not want to pursue subsequent legal action (i.e. tenant who lost her job is willing to leave within a week may receive sympathy). A landlord has to weigh the costs of a lawsuit to recover against the potential recovery. In the alternative, a tenant may find that landlord violated a number of CRLTO provisions. Although section 5-12-140 prohibits (even upon agreement of the parties) the right for the parties to waive any CRLTO rights in the lease, the section does not prohibit subsequent settlement agreements between the parties. Any potential claims may be used as leverage to leave the lease early (if landlord doesn't sue for unpaid rent or early unauthorized termination, tenant agrees to waive any CRLTO claims). Any such agreement should be put in writing and signed by the landlord.
Other Illinois statutory rights
Other laws in the state of Illinois may provide reasons a tenant would not owe rent. For example, the Safe Homes Act (765 ILCS 750/) allows a tenant who is a victim of domestic or sexual abuse to terminate a lease early without subsequently owing the landlord rent. Other state, county or city laws may also allow a tenant to break their lease without penalty.
An attorney should be consulted to see what other possible options a tenant may have and to help advise a tenant on the procedural steps to make the termination effective.