When Do You Use The Singular Or Plural Imperative?

Daginn,

The other day, a friend and fellow learner showed me this picture from his trip to the Blue Lagoon:

The picture is a sign giving guests instructions on how to secure their clothes in the facility's lockers. The reason why he shared is that he noticed that the imperative forms switch between the singular and plural, and he wanted to know why.

In general

The imperative form is the verb form that is used to give people commands. Since you can only speak commands to people that you are talking to, the imperative only exists with second-person forms. You use the second-person singular when commanding one person and the second-person plural when commanding multiple people.

Note: The first-person plural form is also used as a type of imperative. For example, saying "let's go" can be seen as an imperative, "förum!"

Example 1: Segðu mér! - (Tell me!)
Example 2: Segið mér! - (Tell me!)

Þérun

Þérun refers to the old pronoun þér. While þér is still used in modern language as the dative of þú, it used to be its own independent pronoun that was a more formal version of you. Although þér was a singular pronoun used to address one person, it was used with plural verbs and adjectives.

Example 1: Talið þér íslensku? - (Do you speak Icelandic?)
Example 2: Hver eruð þér? - (Who are you?)
Example 3: Eruð þér íslenskir? - (Are you Icelandic?)

The modern use of þérun is reserved to the most formal circumstances, but it is still common in a few phrases, including some commands. For example, the verb að afsaka, meaning to excuse is most often used with the þérun.

Example 1: Afsakið! - (Excuse me!)
Example 2: Afsakaðu! - (Excuse me!)

Instructions

Instructions can seem a little bit more ambiguous. For example, is a cookbook directed only to the reader or is it addressed to the entirety of the potential reading base? The same can be said of a political sign, a manual, or a large host of other things.

Traditionally, these types of mediums use the plural forms because they are written for a general audience. They are addressed to the general readership of the instructions.

Example 1: Bakið í ofni í um 15-20 mín. - (Bake in the oven for about 15-20 min.)
Example 2: Notið viðeigandi hlífðarbúnað. - (Use proper protective equipment.)

However, the singular can be used to be more personal and to address the reader directly. This may be more common in something such as a blog or in a podcast (ex., subscribe to our mailing list!) as compared to a written textbook.

Example 1: Bakaðu í ofni í um 15-20 mín. - (Bake in the oven for about 15-20 min.)
Example 2: Notaðu viðeigandi hlífðarbúnað. - (Use proper protective equipment.)

So what about the pool sign?

Back to the sign. The pool sign uses both singular and plural imperative forms, but it shouldn't. Best practice would have been to choose one form and stick with it throughout. It is written for a general audience by a company in the service industry, so a plural form is likely more appropriate.

As described by an Icelandic native, "It almost feels like the Blue Lagoon poster is written by two people; one who wants to follow the service industry formality and another one who wants to feel intimate and familiar."