M-WA class

The M-WA class is the noun class for people, including the word for 'people':

mtu  -  watu   (person - people)

There are only two nouns in this class that are not people:

mnyama - wanyama  (animal)
mdudu   - wadudu     (insect)

A few M-WA nouns:

mtoto - watoto   (child)
mwanamke  - wanawake  (woman)
mwanamume - wanaume  (man)
msichana  - wasichana  (girl)
mvulana  -  wavulana  (boy)
mwana  - wana  (son/daughter)
mzee  -  wazee  (old man)
mzungu - wazungu (white person)

mwalimu  - walimu  (teacher)
mwanafunzi  - wanafunzi  (student)
mhandisi  - wahandisi  (engineer)
mkulima  - wakulima  (farmer)
mpishi  - wapishi  (cook)

Mwafrika  - Waafrika  (African)
Mtanzania - Watanzania  (Tanzanian)
Mwingereza - Waingereza  (English person)
Mkenya - Wakenya  (Kenyan)
Mganda - Waganda  (Ugandan)
Mfaransa  - Wafaransa  (French person)
Mjerumani - Wajerumani  (German)
Mholanzi  - Waholanzi  (Dutch person)
Marekani  - Wamerekani  (American)

And most other nationalities - you get the idea. The only unexpected one is
Mreno  - Wareno  (Portuguese person)

The reason it's called the M-WA class should be fairly obvious by now.  The same rule applies to adjectives describing the nouns.  The word nzuri is actually -zuri in its most basic form.  So instead of nzuri, words in this class are mzuri in the singular and wazuri in the plural.

Mwalimu mzuri (a good teacher)
Wakenya wazuri (beautiful Kenyans - or good Kenyans, depending on the context).

In the M-WA class the verb prefixes are a- (third person singular, i.e. he/she), and wa- (third person plural, i.e. they).  We've learnt these already, because we were using verbs to talk about people.

Mtoto anakwenda shuleni  (The child is going to school)
Waholanzi wanapenda chizi  (Dutch people like cheese)

A few useful M-WA class words:

huyu - yule        this - that                  Huyu ni mtoto wangu    This is my child
hawa  - wale      these - those             Hawa ni watoto wangu    These are my children
wengi                 many                        Watu wengi sana!     A lot of people
mwengine          other, another           Mtoto mwengine      Another child
wengine             other (plural)            Wazungu wengine   Other white people
wote                   all                             Wanafunzi wote wanasoma    All the students are reading

The most important letter for the M-WA class is W.  W is the prefix for the A of association and for possessives, for example:

mtoto wa jirani                 the neighbour's child
mwalimu wa Kiswahili     the Swahili teacher

mwalimu wake      his/her teacher
wadudu wangu      my insects

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NB.  There are many nouns for humans and animals that are not in the M-WA class.  They take the plural of whichever noun class they are in, e.g. the plural of mbwa (dog) is mbwa.  BUT they follow M-WA class agreements for other things:

Mbwa wa jirani anakuja       The neighbour's dog is coming
Mbwa mkali                          A fierce dog
Mbwa wakali                        Fierce dogs

Noun Classes

Noun classes are very frustrating when you begin learning Swahili, partly because the whole concept is so different from anything we have in English, and partly because it takes time to learn the rules of each class.  

You can't tell what class a noun belongs to if you only know the singular of the noun (although sometimes you can guess), but you can always tell what class a noun belongs to as soon as you know the plural as well as the singular.

There are six noun classes:

1. M - WA

2. N

3. M-MI

4. KI-VI

5. JI-MA

6. U

A lot depends on the noun class, including:

a) Plurals.  So the plural of mtoto is watoto (M-WA), while the plural of kitabu (book) is vitabu 

(KI-VI).

b) Adjectives, including most numbers, e.g. mtoto mzuri (a good child) and kitabu kizuri (a good book). Most of these take the same prefixes as the plural, so they're not very difficult to remember.

c) Demonstratives, i.e. 'this' and 'that', and 'these' and 'those'. 

4) Possessives, i.e. my, your, his, her etc.  The first letter of these words changes depending on the noun class.

5) The A of association (ya, za, wa etc.)

6) Quite a few useful words, including 'many', 'other', 'how many?' and 'all'.  Like the possessives, the main part of the word is always the same, but the first letter or two changes according to the noun class.  E.g. Watoto wengi (a lot of children - M-WA), miti mingi (a lot of trees - M-MI), nyumba nyingi (a lot of houses - N).

7) The person prefixes in verbs, for 'it' and 'they'.  E.g. Yeye anakwenda (He goes - M-WA class), but inatosha (it's enough - N class).

Warning: if you try to learn several of these noun classes at a time it becomes very confusing. Make sure you're confident with the classes you know before you move on to the next one. And if you don't know what class a noun is and aren't sure that you're getting the grammar right - stop worrying, because it doesn't really matter.  Communication is much more important than accuracy, so if you say "watoto nyingi" instead of "wengi" people will understand perfectly well and they won't mind.