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 amount 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.