English as a Foreign Language (EFL) freshman students at the College of Languages and Translation received direct instruction in adjective-forming suffixes, then they took an immediate and a delayed test. Error analysis showed that 36% of the responses were left blank or the subjects duplicated the stimulus word. In 32% they mismatched the word and suffix, in 36% they made spelling mistakes; in 15% they spelled words phonetically, and in 15% they added a noun- or an adverb-forming suffix. Significant differences were found in the amount of errors made by the students on both tests. The number of errors made correlated with the students' vocabulary knowledge. A hierarchy of difficulty in attaching adjective-forming suffixes, faulty strategies used in adjective morphology and possible causes of students' difficulty in adjective suffix acquisition are given.