It communicating — either in speaking the language
It is very different learning a native language since childhood when we learn to think and communicate in a language as a natural process, and when we choose to learn a new language in our youth or as an adult. A native language is part of our being, whereas learning a second language puts a high demand in a time-bound fashion, in learning what should actually take years to master.
One of the biggest challenges in learning a language is attaining fluency while communicating — either in speaking the language or in understanding others/ native speakers when they speak — despite having learnt perfect grammar and vocabulary.
Hence, an effective language lesson needs to address all aspects of a language — Grammar, vocabulary, reading, writing, listening, conversation — in a manner that it can hep the student use the language in daily life, since the objective of learning a language is to be able to communicate effectively.
Learning by oneself and learning as part of a group has distinctly different learning outcomes, Learning alone does not allow the opportunity to understand the progress, however, learning in a group would allow much more opportunity to interact, converse and practice speaking and using the language, and to listen to and try to understand others. Opportunities for tests, quizzes, and assessments allows for comparison and some competition, which can work as a motivator.
One of the most important elements is the ability of the teacher / coach to be proficient in communicating and engaging the entire class.
Though grammar and vocabulary are important parts of learning a language, the ability to use it correctly in writing, speaking and understanding others becomes the most vital thing. Hence an effective method could be learning vocabulary in phrases that can be used immediately(colloquialism, pronunciation, grammar, vocabulary and spelling have to go hand-in-hand.)
Active task-based and practical learning might help in developing the application of the language better and quicker. Other than text book exercises, word games, story books, movies, conversation practice etc. can engage students much better and encourage them to research some grammar and vocabulary on their own for more effective learning and keep them immersed wholly in the learning process. Celebrating any kind of progress also goes a long way to motivate further learning.
An effective language lesson should have a goal & objective for each class, should ideally be a fun and motivating experience, allowing a lot of practice focusing on student talk time, with an attempt to link between the lesson and everyday life. Also effective would be if some parts of the cultural knowledge are also integrated into the learning.