Breve accented characters?

In System Preferences : Keyboard panel : Input Sources tab, click the + symbol to add Romanian Standard. At the bottom, select Show Input menu in menu bar. Quit System Preferences.

When you are ready to type ă , you select Romanian from the menu bar input sources menu, and then press and hold the letter 'a'. Select the ă character from the floating panel:

Then, back at the input source menu bar item, select your normal input language.

A combining macron ā ? You would skip the Romanian language from my previous post, and with a U.S. keyboard, just press and hold 'a'. Select the last element from the floating character panel.

This technique works for all the vowels, except 'y'

For a keyboard shortcut you need to add the ABC Extended input source in system prefs/keyboard/input sources. Then breve is option B followed by the base letter.

But the accent you put on the a in your question is NOT a breve. It is the hachek. For that the keyboard shortcut is option V followed by the base letter.

If in fact you want some macrons, the shortcut is option A followed by the base letter.

Just curious, what language are you typing?