With the spread of English, the conception of English is currently changing into English as a Lingua Franca (ELF) worldwide. However, the form of English teaching and learning is still identified by reference to native-speaker norms. In response to the increasing use of ELF and an emergent need to describe it in various domains, the current study takes a preliminary step in linguistic analysis of the written mode of ELF. Adopting an exploratory case study, the morphological marking of third-person singular present tense main verbs, i.e., -s/es has been analyzed using the Written ELF in Academic Setting (WrELFA) corpus. The selected corpus included 82369 tokens. It was tagged by the LancsBox software, and all instances of variation were categorized into two groups: omission and addition of the suffix. The quantitative analysis revealed that most ELF writers conform to the grammatical rules of English simple-present tense, yet, there is a negligible amount (0.5%) of variation in the use of this suffix in a way that writers either drop it or overgeneralize it. Dropping the third-person singular suffix accounts for around half of the entire variety (56%), and overusing this feature accounts for the other half (44%). Moreover, through qualitative analysis, factors contributing to such variations were identified. The overall findings indicate that such minor grammatical errors shouldn't be overemphasized, as long as they do not hinder the reader's understanding. The results may contribute to constructing a framework for teaching ELF in general and English for Research Publication Purposes (ERPP) in specific.