English proficiency opens doors for many professionals in today's global economy, helping professionals collaborate across borders and pursue career advancements or new paths. For adult English language learners working to improve their English skills and confidence for professional reasons, how effective is one-on-one online language tutoring? This question was central to a 2025 research partnership between Leanlab Education, an independent nonprofit research lab, and Preply, a global language learning platform.
Preply helps people on their language learning journeys by connecting them with real online tutors who provide personalized one-on-one learning experiences. To better understand the impact of adult learners committing to sustained engagement with Preply, Preply partnered with Leanlab to conduct efficacy research, tracking the outcomes of native Spanish speakers working with a Preply tutor for three months to learn English. Does the experience translate into measurable skill gains? Does learner language confidence grow?
The Research
Leanlab followed 67 native Spanish-speaking adults between the ages of 25 and 55 who were learning English to achieve professional goals. Over 12 weeks, each participant completed at least 24 one-on-one lessons with a Preply tutor.
Participants ranged from Elementary to Upper Intermediate proficiency, which allowed us to observe progress from various starting points. To measure growth, we used assessments aligned with the Common European Framework of Reference for Languages (CEFR). This internationally recognized standard gave us a consistent way to evaluate ability across speaking, listening, reading, and writing. Participants also completed surveys assessing confidence in their English language skills and their experience with Preply.
As a third-party research partner, Leanlab brought expertise in codesign research and education technology evaluation to support study design and analysis, with the goal of producing independent findings that are both methodologically sound and useful for Preply and language learners evaluating available programs.
The Findings
The results of our study suggest that with Preply’s one-on-one tutoring model, learners can have a positive and productive experience learning English, and even expedite their learning journey.
Learner Progress
The results showed significant improvement across multiple measures of English proficiency:
One in three learners improved their CEFR-aligned language test scores by a full level after completing 24 or more lessons over 12 weeks.
78% of learners starting at the A2 (elementary) level advanced by a full level, demonstrating particularly strong outcomes for those in the early stages of learning.
Learners showed statistically significant improvements across all skill categories, including speaking and listening, writing and reading, language fundamentals, phonological control, and spoken fluency, grammar, cohesion, and coherence.
These gains happened remarkably fast. Moving up a full CEFR level can take up to 240 hours of study. Many Preply learners achieved this progress in just 24-48 hours of tutoring over 12 weeks—up to 3x faster than typical learning timelines.
Building Confidence
Beyond test scores, learners reported dramatic shifts in how they felt about their English fluency and confidence:
94% felt more fluent after 24 or more lessons.
97% reported feeling confident speaking English.
94% reported feeling confident writing in English.
100% of learners reported making progress toward their goals.
What Worked: Personalization and One-on-One Practice
Learners attributed their progress and positive experience to specific features of the Preply experience:
96% said learning with a real person was key to their progress, highlighting the irreplaceable value of human interaction and feedback.
99% reported having a personalized learning experience with Preply, suggesting that tutors successfully adapted their approach to individual learner needs, goals, and starting proficiency levels.
The research indicates that sustained engagement was critical to achieving results. Learners who completed 24 or more lessons over the 12-week period consistently improved their test scores across all skill areas. The data points to regular, frequent practice with a dedicated tutor as valuable for accelerated English language development.
For English language learners, these findings are encouraging. They suggest that meaningful language skill growth is possible in a realistic timeframe for busy professionals with consistency and personalized, one-on-one support.
For the broader field, this research adds to our understanding of what makes language learning effective. As new tools and technologies emerge constantly, evidence about what works helps learners, educators, and organizations make informed choices. The data from this study underscore the continued value of learning with and from another person.
Looking Ahead
Building on the findings from this study, Preply and Leanlab Education are continuing their research partnership to explore additional questions about effective language learning. This ongoing work will deepen our understanding of how personalized tutoring can best serve language learners with different goals and initial skill levels.
For adult learners considering their options for English language development, this research offers evidence that consistent, personalized one-on-one tutoring can accelerate progress and build the confidence needed for professional success. For the field of language education, it reinforces the enduring importance of human connection in learning—even as technology creates new possibilities for how and where that learning happens.
To learn more about Leanlab Education's independent research services or to explore partnership opportunities, visit our Codesign Product Research webpage. To learn more about Preply's approach to language learning and the results of this study, visit their Proven Progress Platform webpage.