general rule: don't start a sentence with "and, but, so, or then" words. and try not to end with "for now".

Explicitly passing ``encoding='utf-8'`` is recommended if that is what you need as it is the most common text encoding in the world and leaves no room for doubt about your code's intent.

perhaps.

This is tutorial and reader won't know what they need.
I want to teach that UTF-8 is the first choice.

How about this?

``encoding="utf-8"`` is recommended unless you need to use other encoding
because UTF-8 is the de-facto standard nowadays.

How about:
Because UTF-8 is the modern de-facto standard, ``encoding="utf-8"`` is recommended unless you know that you need to use a different encoding.

I'd use "encoding" instead of "codec". I don't see "codec" used anywhere else in this file.

(I didn't rewrite this paragraph at all. I just reflow it.)

How about this?

Appending a ``'b'`` to the mode opens the file in :dfn:`binary mode`.
Binary mode data is read and written as :class:`bytes` objects.
You can not specify *encoding* when opening file in binary mode.

as a :term:`text file`. add the 'a' and no the "for reading and writing" text can go.

We like the encoding to be a keyword for readability so I'd word this similar to "two arguments, often with an encoding keyword when using a text mode" rather than including encoding in the number and mentioning two numbers. it feels more clear to me that way.

At this point, we haven't described the "text mode" yet. It is described in below.

How about "two positional arguments and one keyword argument"?
Since binary file is rare than text file, we can focus on text file at this first open() example.