This study examined the language choice for modality patterns to express the degree of probability specifically in email texts based on Systemic Functional Grammar (SFG) analytical framework. Participants were students enrolled in an English writing composition course at an online-based Korean university. They were required to write an email thanking their professor, in which they stated their future plans (definite or indefinite) with a degree of probability. The text analysis was compared with two groups of students' scores (high-scoring group n=40 and low-scoring group n=29), based on an assessment of the course assignment. After building up two learner corpora, UAM Corpus Tool version 3.0 was used to analyze the language choice closely using a modality system of the SFG framework. The high-scoring group showed more range and frequency in the use of modal verbs combined with a modal Adjunct or another modal expression. Explicit teaching on the importance of expressing the appropriate degree of probability using a range of modality devices, rather than relying heavily on the primary modality (choice of modal verbs) is highlighted as a pedagogical implication.