Word has two classes of 'text form fields' - both of which allow multiple lines. You need to display your Developer Tab on the Ribbon to work with these. All these text data entry fields will wrap to multiple lines so perhaps you are thinking of multiple 'paragraphs' rather than multiple 'lines'.

The old way is using legacy form fields. Hopefully you aren't using these.

The preferred way is to use Content Controls. If you don't require varied formatting inside the Content Control (CC) then you can specify the Plain Text CC. There is a properties dialog for each Content Control and with a Plain Text CC that includes a setting for 'Allow carriage returns (multiple paragraphs)'. If you do require variable formatting inside the CC then you can choose a Rich Text CC which automatically allows paragraphs.

Side Note: The Plain text CC with multiple paragraphs is a misnomer by Microsoft. You type a paragraph mark but it is inserted as a new line break. If you were expecting the new paragraphs to show space before/after and automatic numbers or bullets then you will need to use a Rich Text CC.

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Andrew Lockton
Chrysalis Design, Melbourne Australia