The positioning and relationships of the parts of the phrases:

Consider this: the interjection [introductory to the vocative] "Yâ ayyuhâ" in the phrase "Yâ ayyuhâ al-nâs ubudû - O you people! Worship" appears frequently in the Qur'an since it contains [a number of] fine points. Here the address is corroborated in three respects: by the "yâ - O" which is to arouse; by the "ayyû," [the purpose of] which is to distinguish; and by the "-hâ" [the purpose of] which is to alert. For the address here alludes to three benefits: the replacing of the hardship of the obligation [of worship] with the pleasure of [being] address[ed], and [informing that] the only means of man's progress from the depths of absence to the [high] station of presence is worship. It indicates too that those addressed are charged in three respects: in respect of their hearts, with submission and obedience; in respect of their intellects, with belief and affirming divine unity; and physically, through action and worship.

[The address] infers too that those addressed consist of three groups [the believers, the unbelievers, and the dissemblers]. It hints too at the three classes of the elite, the middle class, and the mass of the people. It betokens too the familiar manner and order of things, which is that a person first of all calls out to a person to stop him, then he recognizes his features and turns to him, then he addresses him and gets him to do something.

In Short: the above subtle points [in the use of the vocative case] corroborate and reinforce the address [by] establishing the mentioned aspects [and meanings].

Concerning the interjection "yâ - O!" since it is addressing the people and they consist of various classes including the heedless, the absent, the idle, the ignorant, and so forth, it both arouses the heedless, and summons the absent, and calls the idle to action, and informs the ignorant, and arrests the busy, and gives direction to those who turn away, and excites the lovers, and encourages the seekers, and urges the perfected to increase [their worship], and it also gives people a jolt.

The distance [inferred] by the "yâ - O!," although the context (maqâm) is one of propinquity [and worship], indicates the grandeur and sublimity of the obligation and its trust (amana al-taklîf).

It alludes too to the distance from when and where the address appeared, of the times and places of those on whom the obligation has been laid.