When I did the Producing Masterclass in 2012, Stephen Follows told attendees to read 100 produced screenplays then 100 unproduced screenplays, and then good writing will jump off the page.
I, like many other screenwriters and producers, have a Dropbox folder with hundreds of produced screenplays, ready to read. I shared these with my HND students. I also have a tracking spreadsheet of which ones I have read.
I can recommend Stephen’s advice: read screenplays.
Are you a writer? read screenplays
Are you a producer? read screenplays
Are you a director? read screenplays
Are you studying film? read screenplays
Are you a DOP? … you don’t need to read screenplays. Go out and shoot something.
But, where do you find these scripts?
Here are a few links…
BBC Writers Room Script Library
for BBC TV, Radio, and Film scripts
The Script Lab
free account to access
The Internet Movie Script Database
Go Into The Story Blacklist.com

When downloading screenplays, make sure you access the .pdf versions, not transcripts.
Transcripts have been written after the film has been released. You want to see what was written before the film was shot.
.pdfs also give you the page number. This helps you to understand structure by page number (especially if referencing some of the scriptwriting gurus with their “this action happens on page number…”)
However, if you just want to read the screenplay online without worrying about page numbers, .htmls will be fine.