master slide text field, change word to bold everything is bold

Are you editing text boxes in the Master Slide view? Since the slides were imported from Powerpoint, there is a chance that those text boxes are placeholder text (the default text that disappears once a user puts their cursor in the final slide). Placeholder text will have all the same styling for each text box.

To apply different styling in a Master Slide:

• Instead of a Placeholder text box, use a normal Text Box, found in the "Insert" tab. This text will not be editable outside of Master Slide view.
• To make editable text, make two placeholder text boxes: one for each line of the paragraph.

This advice is only for Placeholder text boxes in Master Slide view. In the final slide, you will be able to set styling (bold) to individual words/paragraphs inside any text box.