There is no function for adding an accent to typed letters. (macOS High Sierra 10.13.5)

I am not finding accent marks to indicate phonemes as a single character across any OS - they might just have you input the //s manually, since that is already a common keyboard character itself (Unless the symbol you typed is not a / and my computer just isn't interpreting it correctly - you may need to pick up a new font specifically for the characters you seek, but anyone without that font may not see the appropriate characters). For keyboard access to other universal accents/characters you can enable additional input sources (keyboard layouts) via

System Preferences -> Keyboard -> Input Sources -> + (in the lower left corner) and select the language that has the characters you're looking for. This also enables the Input Menu in the menu bar to allow for swapping between different keyboard layouts, as well as a quick way to show the Keyboard Viewer so that you can see which accents/characters become available for your current input source with the current modifier key(s) you are holding down.

If what you're looking for isn't there either, you can set up text replacements for the characters in question if you can copy them from the web. System Preferences -> Keyboard -> Text allows you to replace trigger phrases with any font content you can paste in there (https://en.wikipedia.org/wiki/List_of_Unicode_characters, Apple emojis or symbols). It can be a pain to go through and set up every single letter of the alphabet with the accent(s) you have in mind the first time, but once done it's quick access after that. As an example, you can tell it to convert (w) into ⒲ every time you type it. This allows you to quickly insert foreign characters without having to switch keyboard layouts, too. macOS Sierra: Replace text and punctuation in documents

I find pressing a couple keys directly is the quickest. Option-key-key will assign the accent type the first key represents to the second key, wherein option-e is é, option-a is å, option-i is î, option-u is ü, and option-n is ñ. Hold down option and tap the key that triggers the accent mark you want, release both, then tap the key you want accented.

You can also hold down the key in question long enough to pop up a menu that lets you select which accent you want for it just by itself (this is a fairly recent OS addition).

A more comprehensive list about this stuff can be found at How to use emoji, accents, and symbols on your Mac - Apple Support

• ó – Acute: Hold down OPTION key and then press “e”, then type the letter you want to accent, like é
• ò – Grave: Hold down OPTION key and then press “`”, then type the letter to accent, like ù
• ô – Circumflex: Hold down OPTION key and then press “i”, then press the letter, like ô
• ñ – Hold down OPTION key and then press “n”, then type the letter, like ñ
• ö – Trema: Hold down OPTION key and then press “u”, then type the letter, like ë
• ç – Cedilla: Hold down OPTION key and then press “c”, like ç or Ç
• ø – Hold down OPTION key and then press “o”, like ø or Ø
• å Å – Hold down OPTION key and then press “a”, like å or Å
• Æ – AE Ligature: Hold down OPTION key and then press “‘” like æ Æ
• œ – OE Ligature: Hold down OPTION key and then press “q”, like œ or Œ
• ¿ – Hold down OPTION key and SHIFT key and then press “?” like ¿
• ¡ – Hold down OPTION key and then press “1”, like ¡

Thésé léttérs håvë bëén åccéntëd, yøü knøw¿

rfogarty67 wrote:

For example, one of the letters I need to accent is /w/, and this is impossible

This can be done via the ABC Extended (formerly US Extended) keyboard layout. See this chart for the option key shortcuts:

http://sites.psu.edu/symbolcodes/mac/codemacext/

rfogarty67 wrote:

I will look for a font to download, since I had been trying to use the 'Symbol' font for writing Greek, but it seems I will have to download a separate font after all.

Fonts are really irrelevant to the problem of adding accents, and it would be a mistake to use an old Symbol font for writing Greek, because it is a non-standard encoding. Instead use any of the many greek capable fonts contained in MacOS already and use the Greek input source or the Character Viewer (Edit > Emoji and Symbols).

Does High Sierra still have the Character Viewer which is in the earlier systems?

of course that may not include the particular diacritical you want.

You're welcome. Have a nice day.

Perhaps you need keyboard for the language you are typing in. What language is that?

"...The fault for this lies with Apple (I am sure it would be very straightforward and intuitive with a Microsoft computer/OS)..."

What is the Windows keyboard command you'd use to accent a w?