Morphology is the study of meaningful units. A morpheme is the smallest unit of meaning, and it combines with other morphemes in order to create words. Some morphemes, however, are able to stand alone as a word. For example, in English, the word "dog" consists of a single morpheme, which can stand alone. However, the word "dogs" consists of two morphemes, the second morpheme being the plural marker "-s". Because "-s" can be taken to mean plural, it is a morpheme. However "-s" cannot stand alone. Morphology essentially looks at how new words are formed in a particular language.
There are two processes for word formation. These are derivation and inflection. Derivation creates new words that are different in meaning to the original word. Often the two words belong to different word classes or parts of speech. Consider the words "teach" and "teacher". Through adding the derivational morpheme "-er", "teach" (which is a verb) becomes "teacher" (which is a noun). Inflection, on the other hand, involves the addition of grammatical information to a word. The conjugation of verbs is an example of this, as is taking a singular noun and making it plural.
Morphology features in sign language as well. Signs can be treated as individual words, although they typically consist of several morphemes. Spoken languages typically involve affixation (which is the adding of suffixes, prefixes, infixes or circumfixes) in order to form a new word, rather than changing the root word. Sign languages, on the other hand, involve changing the internal structure of a sign, rather than adding onto a sign.
In sign language, a number of nouns are derived from verbs. This also happens in English. The word "progress" is an example of this. When the word is used as a verb, the stress falls on the second syllable (ie. progréss). In order for the word to become a noun, the stress is shifted to the first syllable (ie. prógress). Another way of deriving nouns from verbs is to add "-er" as can be seen from "teach" and "teacher". With regards to sign language, there is are similar processes involved. If you consider FLY and AIRPLANE in American Sign Language, you will notice that both signs have the same handshape and orientation. The location is usually also the same in related verbs and nouns. The main difference, however, lies with the movement used. Another example is SIT and CHAIR. Like English, a suffix that resembles "-er" is sometimes used in sign language. This can be seen from TEACH and TEACHER. The first part of TEACHER is the same as TEACH, however, there is an additional part, which could be seen as being a suffix.
Compounding is a word formation process used in derivation, which involves combining two separate words to form a new word. The new word generally has a similar meaning to the original separate words. For example, "green" and "house" can be combined to form the compound noun "greenhouse". However, sometimes the meaning can be completely different. Compounding also happens in sign language. For example, in Australian Sign Language, BELIEVE is derived from THINK and TRUE.
There are several ways to show number in sign language. A sign is sometimes reduplicated (repeated) in order to show plural. For example, in French Sign Language, the sign for ANIMALS involves a more drawn out version of the sign for ANIMAL. Or, in the case of a sign that has more than one segment such as BOOKS, you would just repeat the sign twice to indicate two of them. Another way of expressing plural is double articulation. A single sign such as AEROPLANE would be signed using both hands. The right hand would move towards the left, while the left hand would simultaneously be moved towards the right. If, however, you wanted to be more specific and sign something about "five aeroplanes", then you could sign AEROPLANE FIVE. Typically adjectives come after nouns, although this may vary from sign language to sign language.
Verbs in particular can be modified in order to convey more information about how the action took place or the state of completion of the action. This is called aspect marking. Take the British Sign Language sign for WALK for example. If you repeated the sign, this would indicate that you walk often. If, on the other hand, you perform the sign with a jerking movement, it indicates walking awkwardly. Performing the sign with a stop-start motion suggests that the action unfolded bit by bit. So, for example, as one walks, one stops to look at the flowers before carrying on. If you change the speed of the hand movement, it would correspond to either walking fast or slowly.
In addition, there are three types of verbs in sign language, which are referred to as plain, spatial and agreeing verbs. Each of these typically involve a different usage of space in order to convey grammatical information about the verb. With regards to plain verbs, location is part of the sign itself, and does not have a separate meaning. However, changing the location of the sign results in the meaning of the sign changing. Plain verbs also don't reveal any information about the subject or object of a sentence. Examples from Spanish Sign Language include FORGET and KNOW. Agreeing verbs include information about the subject and the object through changing the direction of the movement of these signs. Consider I-HELP-YOU, YOU-HELP-ME and I-HELP-HIM/HER. The initial position of the sign would be nearest to the person who performs the action (it. the subject), while the final postion of the sign would be nearest to the person who is affected by the action (ie. the object). Movement also plays an important role in spatial verbs. Spatial verbs show the location and movement relating to the action. An example, would be PUT from Australian Sign Language. While signing this, one would usually give an indication of where one actually put the object one is referring to, whether one puts it on a surface, far away from one's self or even putting the object under something.
Lastly, sign language makes use of classifiers. A classifier is used to represent a class of objects that are usually combined with movement and sometimes location as well as non-manual features, and in doing so, forms a predicate. A classifier can be likened to a pronoun in the sense that classifiers can stand in place of a noun. Classifiers are not as common in spoken languages. Classifiers typically resemble that which they represent (also called a referent) in some way.
The following classifiers are used in South African Sign Language:
i) Single finger G: used for long, thin referents, such as man, woman, pencil, toothbrush.
ii) Flat hand B: used for 2-dimensional referents, such as car, bed, table, books, wall, plate as well as other flat surfaces.
iii) Curved, clawed 5: used for 3-dimensional referents, such as rock or cake.
iv) V: used for things coming in pairs, such as eyes, legs, people.
Classifiers are useful in verb constructions such as CAR-TRAVELS-UP-WINDING ROAD. The sign for CAR would first be signed, and then the flat hand B would then be used to convey the rest. One would tilt the active hand upwards and move it from side to side as the active hand is moved upwards. Classifiers differ from sign language to sign language. American Sign Language, for example, has its own classifier specifically for vehicles, which is a 3 handshape.