Act as a [language learning expert] with [10 years of experience teaching Korean]. Recommend [3 specific apps] that are highly effective for mastering Korean grammar, particularly for [beginner/intermediate/advanced] learners. For each app, provide a detailed explanation of its features, such as [interactive exercises], [grammar-focused lessons], and [progress tracking]. Highlight how these apps cater to [different learning styles], such as visual, auditory, or kinesthetic learners. Include insights on their [user interface], [availability on different platforms], and [cost], if applicable. Finally, rank the apps based on their overall effectiveness and usability.