Is it possible for us to create a template outside of AEM form first and import it into AEM forms after we finish it?
if yes, which coding language and which format must we use?(can we use HTML, C#, C++. is it only possible with PDF or can we use other tools?)
If no, what can I do to make my files compatible with AEM forms other than to use AEM to build the form?
Creating a template in AEM or even AEM forms is as simple as creating a page in a language you best know. Creating template outside the product and importing it might not work and is not recommended. However, if you have JS and CSS files that you want to re-use can be very well done (may require some updates). OOTB template editor(Highly recommended) further eases the template creation but you can, of course, do it via CRX de as well. I have compiled few documents and video link that will help you with purpose.
Let us know if you need any info further.
does this mean if i put JS and CSS into AEM i can use those in AEM forms too(like if i made a component for the form to use aem will know about it)? I have not touch AEM so i'm not sure about this and when you say may require some updates, is that update meant for the CSS and JS? or is it meant for something else?
Adding further, you can also use Eclipse for developing AEM based projects.