The present paper deals with the Slovak secondary school students' problem of using commas in the complex sentences containing adverbial and nominal subordinate clauses while writing essays in English. It also concentrates on the English teachers' ability to see correct/incorrect usage of commas in their students' works. The research sample is formed by school leaving exam essays written by the students whose English is at an B1 level. The digitalized texts of 115 essays are uploaded in The Main Corpus and divided into 8 different subcorpora referring to different essay's topics. Detailed corpus linguistic and statistical analyses lead to the conclusions that the overall commas use success of the observed EFL students directly depends on how difficult the topic of an essay is, and that the students' error rate in adverbial clauses is much higher than the rate in nominal clauses. The last finding referring to the fact that English language teachers overlook or incorrectly correct commas in a substantial number of sentences makes the authors think about all the possible roots and reasons of the students' inaccuracy in commas usage in their written performance and several suggestions how to solve this problem.