It is six years since I started to study English.
I thought that 初めて was the right way to write it when it implies “for the first time”. Is there a reason the other version is used? Is there some other reason for this form to be better here?
No. You cannot replace 始めて with 初めて in this sentence structure.
- 初めて (for the first time or firstly) is an adverb, and 初めての (first-time) is an adjective.
- 始めて as in 始めてから, on the other hand, is a gerund (verbal noun) meaning “starting”.
私は英語を「初めて」からもう6年になる = Six years have passed since firstLY my English.
==> Since firstLY is grammatically incorrect and sounds very unnatural so as 初めてから in Japanese
The original Japanese sentence can be split into two:
- (Natural) 私は英語を始めた。それからもう6年になる = I started (learning) English. Since then, six years have passed.
- (Incorrect) 私は英語を初めた。それからもう6年になる = I English firstly. Since then, six years have passed.
The latter is missing a verb. That’s why your proposed sentence is grammatically broken.
If you really want to use 初, you can say
- 私が初めて英語を習い始めてから、もう6年になる。= Six years have passed since I started learning English for the first time.
- 私が初めて英単語を覚えてから、もう6年になる。= Six years have passed since I could memorize the first English word.
As you can see, 初めて as an adverb requires a verb such as “to start” or “to memorize” etc.