Why do they use 'multe' instead of 'multo'?

Question

I thought 'multe' is an adverb and 'multo' is a corresponding noun.

In this sentence,

Ili vendas multe da spicoj kaj oleoj

it looks quite weird to me why an adverb multe is used instead of a noun 'multon'.

Answer 1

The basic word is the adjective multa for which PIV gives three definitions (my translations; all examples from Zamenhof via PIV):

1. big undefinied part of some large set which cannot be expressed with a number: la tro multa parolado lacigas lin
2. (in plural) big undefinied set of enumerable things in relation to a small set: el multaj milonoj fariĝas milionoj
3. (in plural without a noun) a large group of people: multaj serĉas favoron de reganto

From the definition 2 follows that you can say Ili vendas multajn spicojn kaj oleojn.

The adverb multe also has three definitions:

1. denotes a long-lasting or a repeating event: ne multe dormi
2. denotes an event which concerns a large group (of something) or is relative intense: kiu multe profitas, ankaŭ perdon ne evitas
3. emphasises the comparison: akiro de saĝeco estas multe pli bona, ol oro

You connect a quantifying adverb to a noun using the preposition da, and thus you can say Ili vendas multe da spicoj kaj oleo.

There is also the noun multo which has only one definition: a large group or quantity: mi neniam prenas kun mi multon da pakaĵo.

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From the definitions follow that the adjective describes the quantify, the adverb the action and the noun stands for the quantity itself.

• Ili vendas multajn spicojn kaj oleojn. : They sell many spices and oils.
• Ili vendas multe da spicoj kaj oleoj. : They sell lots of spices and oils.
• Ili vendas multon da spicoj kaj oleoj. : They sell a lot of spices and oils.

The first one speaks about large selection while the other two of big sales volumes. When it comes to the last two cases, multe and multo are interchangeable, but for some reason the latter is very rare. I have seen it mostly without a noun (e.g. Ne restas multo por fari.).

Note that multe and multo are not always interchangeable. For this you might want to take a look at Marcos Cramer's answer to What is difference between "multo" and "multe"?