how do I open a .txt file ?

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.

Triple-click the line below to select it, then drag or copy into the Terminal window — do not type — and press the space bar:

file -b

Now switch to the Finder and drag the item(s) into the Terminal window. More text will be added to what you entered.

Click in the Terminal window to activate it, then press return.

Post any lines of output that appear below what you entered — the text, please, not a screenshot.

What program is opening the file when you doble click? Are you sure the file is a text file. Even though it has a .txt extension maybe whoever sent it to you named it .txt in error. Maybe they sent you a Word document and named it with a .txt file extension. If this is the case rename the file to .doc and open it with Word.

Click on the file and select get more info. See what is the default application identified to open the file. If the file is actually a text file make sure textedit or another text editor is set as the default application.

The file is likely not a text file. Who ever named it named it incorrectly with a .txt file name. Where did you get the file? Try renaming the file to something.doc and try opening it with LibreOffice.

It sounds like a binary not a txt file. These are executable program files (similar to .exe on windows). They're often scripted in text editors so that may be why the file has the wrong extension.

The real question here is who sent you the file and why? What is it supposed to contain? That's where you need to start to get answers.

Why can't you open it when you double click on it? What happens when you try? Any text editor will open it. Try TextEdit.