Eu particularmente gosto da Cambly, mas se tu não tem nem os fundamentos básicos e um vocabulário mínimo, não recomendo. Cambly é mais conversação, o único ponto negativo deles é ser apenas 1 hora por semana.

No mais, se você quiser construir uma boa base de estudos, há cursos muito bons no coursera que eu fiz, inclusive de graça. Segue a lista:

Nível básico:

• [] https://www.coursera.org/learn/word-forms-simple-present-tense/lecture/STPUG/meet-your-teachers
• [] https://www.coursera.org/learn/questions-present-progressive-future-tenses?specialization=learn-english-beginning-grammar
• [] https://www.coursera.org/learn/simple-past-tense?specialization=learn-english-beginning-grammar

Nível intermediário:

• [] https://www.coursera.org/learn/perfect-tenses-modals?specialization=intermediate-grammar
• [] https://www.coursera.org/learn/adjective-clauses?specialization=intermediate-grammar
• [] https://www.coursera.org/learn/tricky-english-grammar/home/week/1

Nível avançado:

• [] https://www.coursera.org/learn/verb-passives?specialization=advanced-grammar-punctuation
• [] https://www.coursera.org/learn/conjunctions-connectives-adverb-clauses?specialization=advanced-grammar-punctuation
• [] https://www.coursera.org/learn/noun-clauses-conditionals?specialization=advanced-grammar-punctuation

Cursos extras:

• [] https://www.coursera.org/specializations/improve-english#courses
• [] https://www.coursera.org/specializations/speaklistenenglish#courses
• [] https://www.coursera.org/specializations/learn-english#courses
• [] https://www.coursera.org/specializations/academic-english
• [] https://www.coursera.org/specializations/advanced-grammar-punctuation