The current study focused on acquisition of If-conditionals by L2 learners of English with two different native tongues, Spanish and Korean, and with two different proficiency levels, high and low. An experiment with two subparts, each focusing on production and comprehension respectively, was performed in order to explore which factor among input frequencies, grammatical complexities and L1 influence best explains the acquisition of If-conditionals by L2 learners. The results from both production and comprehension data suggest that If-conditionals with lower hypotheticality are acquired before than those with higher hypotheticality by L2learners, just as they are by children learning their L1, and hierarchies of input frequencies best explain the acquisition order of If-conditionals. At this point, it is not possible to distinguish between influences of input frequencies and typological universals, as they coincide. One thing for sure is that both L1 and L2 acquisitions seem to be influenced by them, suggesting the universality of language acquisition. Influence of instruction, on the other hand, was also found, especially in the production data of the higher-level L2 learners. Even though there were no differences in the acquisition order of If-conditionals between the two groups of learners with different L1 backgrounds, the influence of L1 was also found in the analysis of non-target forms produced by learner participants.