Dan Thies: Unless there’s a shopping cart (Magento, Woocommerce, etc.) running on the primary domain. Then subdomain for sure.
Hossam Eid: Dan Thies Am adding another lanaguge to my websote (conntnet based site ) but the thing not all articlrs will be bilingual Some articls will be only avl in English Others in Arabic So should i go for en.mysite(dot)com or mysite(dot)com/en
Michael Martinez: To a search engine there is no difference. Both are URLs. The only difference for search engine optimization is what you do with the URLs. If you treat the subdomain URLs the same as you would treat subfolder URLs, you won`t see any difference in performance. HOWEVER, subdomains are usually managed as separate Websites by the Web hosting servers. It *IS* possible to serve multiple subdomains from a single host (WordPress multisite networks do that). But if you`re running distinct software on the subdomain (such as a shopping cart, to borrow from Dan`s examples), then you really should set up a separate host for the subdomain. Bottom line: Keep it as simple as possible and don`t get bogged down in senseless debates about "which is better for SEO". Neither is better. Neither is worse.
Travis Bailey: A wise man once told me that either is fine, but subfolder has always been right - at least for pages you intend to rank. May as well go for what`s always been true. Though I don`t see Google changing their mind now.But if I were to use a sub, that would be for things that I generally don`t want indexed/ranked. Like some employee/associate portal.
Michael Stricker: What you should focus on instead would be spotless hreflang implementation.
Dave Elliott: I`d use a subfolder unless you have a good reason not to....I have had a few clients where sub domain was a better option but it`s rare.