In general, we use 'the', which is known as the definite article, when we are referring to one specific thing. 'Definite' means 'determined or resolved' or 'free from any doubt'. When we use the definite article, there should be no doubt about what we're referring to.
But, like anything else in English, it's never that straightforward. Here are some more elaborate rules from the Purdue Online Writing Lab.
As an example of how nuanced the usage of 'the' can be, look at this English Language Stack Exchange question and answer.
Learning when to use an article, and which one, is one of the most difficult parts of learning the English language. Here's a paper I ran across detailing these difficulties for ELLs.
You've correctly chosen not to include an article in your example sentence.
This is good, because this was a tricky decision. 'Format' is in a class of words known as 'weakly countable' that doesn't get talked about much. These words are tricky, because sometimes they take an article, and sometimes they don't. The PDF article I linked gives this example:
e.g., She likes beer. (But one can still say ‘I would like to have
When it comes to 'format', the main thing is whether you're distinguishing between several different formats. In your sentence, you're really describing the list, rather than talking about the format. Another issue is that there actually is no specific 'text format'. There's ASCII, Unicode, and so on. So I would normally use 'a' with 'text format', not 'the'. Let's use 'ASCII' for an example with 'the':
The ASCII format results in very small file sizes, unlike the Word document format.
I made a file in ASCII format.
Avoiding the Issue
You can often rephrase a sentence to avoid this issue:
For example, to teach the distinction between short and long vowels like /ԑ/ and /iː/ (as set and seat) or /a/ and /a:/ (as got and dart), an appropriate text-formatted list can be prepared.
"in the text format"on Google Books, Google returned "About 947,000 results (0.22 seconds)". However, when I reached page 28, it said "Page 28 of 274 results (0.37 seconds)". That's how unreliable the estimated number of results is.