I am creating a new site with Magento 2.1.8. I need to import only the products from an old site that is running on Magento 1.9. I want my old site to still be fuctional even after copying the product data. Please help!
You need to first take a backup of your current running 1.9 site's database. You can use migration tool , full guidance of migration process provided on this link.
This tool provide functionality for migration of products.
As you want only want to copy product from magento1 system,Then my suggestion, export all product from magento1 using Default Data flow export or Export.
Then import then them to magento2 using default import.
It's highly recommended to use Official Magento Data Migration tool which will allow you migrate products, customer and more!