For example, you can ensure that appropriate spaces are inserted when you copy and past text. So, if your cursor is at the end of a word, and you want to paste another word after it, you don't want to have to type a space first. This is what Smart Copy/Paste is for. Make sure it's checked to have the correct spaces inserted automatically.
Smart quotes are curly quotes; you may or may not want to use them. If you're typing a term paper, they're certainly appropriate; but if you're typing HTML code, they're not.
Smart dashes allow you to have two or three dashes converted to an em-dash. So when you type —, your text editor will replace those characters with —.
If Smart Links is checked, then any URL, such as www.macworld.com, will become clickable; you'll see a line under it, and you'll be able to click it to go to that webpage.
Data Detectors can spot certain types of texts — dates, email addresses, and so on — to allow you to perform actions on them. When this option is selected you can, for example, create a new Calendar event by hovering your cursor over a date, then clicking the downward-pointing triangle. Similarly, you can add an email address to Contacts by hovering and clicking the triangle.
Text Replacement is a really useful tool that lets you set up snippets of text that are automatically replaced by longer words or phrases. You can have, say, sy replaced by Sincerely yours, or you can have (c) replaced by ©. However, for these replacements to happen, you need to add them to a list first: Open System Preferences, click Keyboard, then click the Text tab. Click the add button (+) to add a new replacement to the list.
It's worth noting that these replacements can sync across all your devices via iCloud. So you can set up replacements on your Mac and access them on another Mac, as well as on your iPhone or iPad, if you're syncing them all via Apple's Web service. (On iOS devices, you can see these shortcuts in Settings > General > Keyboard; scroll down to the Shortcuts section.)
While you can turn the above substitutions on or off for your app in general, you can also apply them to a document or a selection. To do this, choose Edit > Substitutions, then Show Substitutions. A floating window will display.
You can turn the various substitutions on or off, but you can also apply them to your entire document (Replace All) or just a selection (Replace in Selection). The Text Preferences button lets you quickly access the Text Replacement settings I discussed above.
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