All languages have a complex grammar, David Crystal
To paraphrase Steven Pinker, we are born with the capacity to learn a language — regardless of how difficult it may appear in a textbook. A child growing up in Japan will learn (spoken) Japanese as easily as one in France will master French.
Learning a secondary language, we can compare the relative difficulty, though as the linguist David Crystal points out, 'all languages have a complex grammar
there may be relative simplicity in one respect (e.g., no word-endings), but there seems always to be relative complexity in another (e.g., word-position).
Nonetheless, the relative flexibility of English is very helpful for beginners.
What helps English language learners?
The most obvious advantage is that English is what linguists call a 'weakly inflected' language. It has a comparatively simple morphology (word formation).
This means that the form of a word changes less frequently than in more inflected languages.
And though there are conventions, these do not amount to a rulebook. The previous sentence ignored one in starting a sentence with a preposition - now standard journalism practice.
Nor is there an English-language academy policing usage.
Words, Words, Words
English may seem 'easy,' but bear traps await. One is irregular verbs - see here. Another is the use of the definite article. Compare I teach English in the prison, with I learned Spanish in prison. Ditto for in the hospital and in hospital
Then there is the sheer size of the English lexicon or vocabulary (those twenty volumes of the Oxford English Dictionary) and the vast number of phrasal verbs.
Without getting into the technicalities, a phrasal verb changes meaning by adding a preposition to the root verb. These are maddeningly illogical for English language learners - see here