This study investigates the relationship between foreign language aptitude and the learning of two English structures defined as easy or difficult to learn. Using a quasiexperimental design, 66 secondary-level learners of English as a foreign language from three intact classes were provided with four hours of instruction on the "passive" (a difficult structure) and the "past progressive" (an easy structure). Language aptitude was measured using the LLAMA Aptitude Test (Meara, 2005). Language outcomes were measured with a written grammaticality judgment and an oral production task. The results revealed that one of the aptitude components, grammatical inferencing, contributed to learners' gains on the passive but not the "past progressive" on the written measure. Another component of aptitude, associative memory, contributed to learners' gains on the "past progressive" on the oral measure. The results provide support for the claim that different components of aptitude contribute to the learning of difficult and easy L2 structures in different ways. There is also support for the proposal that different components of aptitude may be involved at different stages of language acquisition (Skehan, 2002).