Tom must observe and perform only the third covenant. The burden of covenants does not pass at common law. The benefit of the first covenant has passed to Sarah in equity due to the use of words of annexation, but the burden does not pass to Tom as the covenant is positive. The burden of the second covenant has passed to Tom under the rule in Tulk v Moxhay, but the benefit has not passed to Sarah as no words of annexation have been used. Words of annexation have been used to pass the benefit of the third covenant to Sarah, and although it is positive in nature and would appear to violate the rule in Tulk v Moxhay, the rule in Halsall v Brizell applies as Tom derives a benefit in the use of the driveway, thus the burden of the covenant passes. The fourth covenant is personal and does not touch and concern the land, therefore it cannot be enforced by Sarah. 

Key Point: The enforceability of covenants depends on whether they touch and concern the land and the rules governing the passing of benefits and burdens. Positive covenants generally do not run with the land at common law, but equitable principles and specific rules, such as those in Halsall v Brizell, may allow certain covenants to be enforced if they provide a direct benefit.