It sounds like your form is not protected for filling in forms. When it's not
protected you can type anywhere in the document. (In this case, you're
actually typing in the space immediately before the field, rather than in the
field itself.)
To fix it, you need to either protect the document as a form OR make sure
you've actually selected the text field, rather than simply placing your
cursor before the field.
The easiest way to protect the document is to use the Forms toolbar and
click the lock icon. Another way (more complicated) is to use Tools - Protect
Document - Allow only this type of editing - Filling in Forms.
To select the text field, drag over it until it turns black (you probably
have your options set to show field shading always, rather than only when
selected). Once it turns black, you can type there, but the text field will
disappear and be replaced by normal text.
In an unprotected document, the only easy way I know of to add text to a
text field and leave it a field is to double click on the field and then type
in the "Default Text" window.
I hope this long discourse helps.
Fred