Just in case you only see one post at a time on a thread but want to read the whole thread with all the posts in chronological order, here is what to do:
Just click on the top right of the beginning of the thread, on the box marked "Linear mode". (New members tend to get thread mode by default). This will change the view temporarily.
To change the view to linear permanently, you need to go to:
User control panel;
Edit options (in "Your profile" section);
Thread view options;
Thread view mode - select "Linear mode" in drop-down box
Just click on the top right of the beginning of the thread, on the box marked "Linear mode". (New members tend to get thread mode by default). This will change the view temporarily.
To change the view to linear permanently, you need to go to:
User control panel;
Edit options (in "Your profile" section);
Thread view options;
Thread view mode - select "Linear mode" in drop-down box