I don't seem to be able to find the search function. How do I search for questions with a certain tag or containing a certain string? I use an iPhone. I'm sure I used to be able to access "search".
Are you using the app or the mobile web site? On the mobile web site, next to the "All Questions" there is a very unobtrusive magnifying glass that took me a while to find. A search box will pop up, but there's no link to remind you of the advanced search syntax.
Here is some of the syntax:
[tagname] - limit results to a certain tag
is:answer - post type
user:me - find posts by you
body:"apples oranges" - limit words to titles or the body of the post. Only questions have titles, so if you use titles you will only get questions back.