If you translate software, sooner or later you run into strings like this one from the Galician translation of WordPress core:
Enlarged image %1$s of %2$s: %3$s
Or this one from Firefox on Mozilla Pontoon:
Close { $tabCount } tabs
Those %1$s and { $tabCount } fragments are placeholders: marks that the program replaces with real content when it runs. They are a common source of doubt for volunteer translators, and mistakes in them can break what users see. This guide covers the main placeholder families, plus other strange symbols (&, _, @, #) that aren’t always placeholders but can cause just as much confusion. The examples come from free software projects with active Galician translation teams, mainly WordPress and Firefox, but the basic principles are useful when translating into any language.