- At the bottom of the device is a microphone and a microUSB port for data.
The way the information is organised in this sentence is very interesting. If you look at it quickly, you may think that at the bottom of the microphone is the subject of the verb be - but it isn't! When we use constructions like this, we rearrange the parts of the sentence for a special reason. Afterwards, it isn't always easy to see what function the different parts of the sentence have. The grammar of the sentence is perfectly correct, though.
Let's take a closer look at how the sentence works. The example you give is a bit complicated, because it includes a co-ordination with and (a microphone and a microUSB). So here are some simpler sentences with the same general structure:
- At the bottom of the garden is the large shed.
- At the bottom of the garden are the large sheds.
Verbs in English agree with their subjects for number and person. Here we can see that the verb be is taking the third person singular form is in the first example, and the plural form are in the second. The only thing that is different about the two sentences is that in 1) shed is singular and in 2) sheds is plural. This seems to suggest that the large shed(s) might be the subject.
Someone might complain about this: At the bottom of the garden is not really a proper 'thing'! Maybe it is the subject, but we can use it with either a third person singular or a plural verb form. This is a quite reasonable theory, so let's test it:
- At the bottom of the garden are the shed. *
This is definitely very wrong indeed. In this construction the verb must agree with the noun phrase that comes after it. This is strong evidence that the large shed is the subject in sentence 1) . Some more evidence for this is that sentence 1) gives us exactly the same information as:
- The large shed is at the bottom of the garden.
This sentence has all the same words as sentence 1, and it has the same literal meaning. In this sentence the large shed is definitely the subject!
The rest of the sentence.
If the large shed is the subject of 1), what is at the bottom of the garden? In your question you described it as an adverb. It looks like what some people call an adverbial phrase, but it doesn't have the same function in this sentence. Compare these two examples:
- I play football every day at the bottom of the garden.
- The shed is at the bottom of the garden.
In the first sentence at the bottom of the garden is giving us extra information about the action described in the sentence. It is not essential. It is not important for the structure of the sentence either. We can take it away and the sentence is still fine. So here at the bottom of the garden is an adjunct - it behaves like an adverb. In the second sentence though, at the bottom of the garden seems to be essential information. And if we take it away, both the meaning and the grammar are bad, as we can see:
- I play football every day. (correct)
- The large shed is. * (wrong)
In the second sentence the verb be needs a complement - another phrase to complete the sentence. If we don't have it, the sentence is badly formed. Here, at the bottom of the garden is the complement of the verb be ( - in the same way that cheese is the complement of the like in the sentence I like cheese). It is not behaving like an adverb.
Your sentence is an example of subject-complement inversion. This means that the subject moves to the end of the sentence and the complement moves to the beginning. It is very common with complements that tell us where something happened or where something is. With the verb be this kind of inversion is only possible with a complement and not with an adjunct (read adverbial phrase). Compare:
- My only friend is in the garden / In the garden is my only friend. (correct)
- My only friend is happy in the garden / In the garden is happy my only friend. (wrong)
With other verbs we can sometimes do subject-adjunct inversion, but only in very restricted environments. It is much more rare. Here is an example though. Notice that there is no auxiliary verb necessary here. - this is not subject-auxiliary inversion:
- Five years later came the final straw.
When do we use subject-complement inversion?
We normally use this kind of inversion because we want to put the new or interesting information at the end of the sentence where it has more emphasis. Sometimes we do it because we want to link the complement with something we have already been talking about. However, if we have already been talking about the subject but we haven't been talking about the complement, we cannot do subject-complement inversion:
- There was a huge garden outside the palace. In the garden were three grizzly bears. (correct)
- There was a huge garden outside the palace. In front of the long hallway was the garden. (X)