Why Offers Are Always More Than Just Offers
1. In English, there is no congruent lexicogrammatical realisation of the speech function 'offer' in terms of mood (see Halliday 1994: 95; Halliday & Matthiessen 2004: 110).
2. Consequently, all offers involve interpersonal metaphor.
3. Grammatical metaphor is a junctional construct: the grammatical wording simultaneously realises both the congruent and the metaphorical meaning (Halliday & Matthiessen 1999: 263, 271, 283).
4. On the semantic stratum, the congruent meaning is realised as the metaphorical meaning (Halliday & Matthiessen 1999: 288, 293).
(a) an offer that is realised as an imperative clause is simultaneously an offer and a command, where the offer is realised as a command (see, eg, Halliday & Matthiessen 2004: 139 on suggestions);
(b) an offer that is realised as an interrogative clause is simultaneously and offer and a question, where the offer is realised as a question;
(c) an offer that is realised as a declarative clause is simultaneously and offer and a statement, where the offer is realised as a statement.
See an analysis of a clause complex with imperative and declarative realisations of offers (the one that triggered the discussion) here.
See also the semantic system of speech function (Halliday & Matthiessen 2004: 108) for the valeur of initiating and responding moves within the overall system.