Conditional logic can be turned on in the advanced settings of each of your fields. This allows the hiding, or showing of that particular field when certain conditions of other fields are met.
This rules settings will “Show” this field on the form only if “Tim” (case sensitive) is entered into the First Name field and somewhere in the Comments field the word “Yes” is contained.
You can do the same to “Hide” a field as well under certain scenarios. This is useful for a complex form that only needs to show relevant sections if certain options (potentially in a drop down or choose one field) are selected.