Contracts
Formation
A valid contract needs competent parties, mutual assent (offer plus acceptance), consideration, and a lawful purpose. Land contracts must also be in writing and signed by the party to be charged (Statute of Frauds; Indiana's version is at IC 32-21-1-1). Missing an element can make an agreement void (illegal purpose, incompetent party) or voidable (minor, fraud, duress).
Offer and acceptance
A counteroffer rejects the original offer and creates a new one; an acceptance that changes terms is really a counteroffer. Offers can be revoked any time before acceptance, lapse by time or rejection, and an option keeps an offer open because it is supported by separate consideration.
Contract types to recognize
Option, right of first refusal, land (installment) contract where the seller keeps legal title while the buyer holds equitable title, lease, listing agreement, and buyer-broker agreement. Contingencies - financing, inspection, appraisal, sale of the buyer's home - must be satisfied or expressly waived, and "time is of the essence" makes deadlines enforceable.
Traps
Earnest money is not liquidated damages unless the contract says so. "As-is" does not waive fraud or required disclosures. Assignment transfers rights; delegation transfers duties. Know the remedies: damages, rescission, specific performance (land is unique), and liquidated damages if reasonable. Watch merger - contract terms merge into the deed at closing.
Rule of thumb
If the answer turns on offer/acceptance, contingency deadlines, or remedies, it is a contract question. If it turns on who represents whom, it is an agency question.