You can show or hide a field based on what the client enters in another field.
Go to Leads, open the Forms tab and open your form.
On the Build tab, click the gear on the field you want to show or hide, then click Conditionals.
In "If", choose the field to check, pick an operator and enter or pick the value.
To check more than one thing, click Add new condition.
Choose Show or Hide.
Operators you can use:
Text fields: is, is not, contains, does not contain, starts with, does not start with.
List fields: is, is not, is any of, is all of.
There is no "is blank" or "is empty" operator. A field also can't use itself as its own condition.