1. One function of se- with nouns is to form adjectives that mean “sharing (base), having the same (base)”, where the base is a noun.
Examples:
Dia teman sekelas saya. He is a classmate of mine.
Maya rekan sekamarnya. Maya is his/her roommate.

Alternatively, the se- (base) form may occur as a predicate and be followed by an accompaniment phrase beginning with dengan “with”.
Examples:
Wanda sekelas dengan Yolanda. Wanda is in the same class as Yolanda.
Anaknya seumur dengan anak saya. His/her child is the same age as my child.

Other words in this group include:
searah (dengan) having the same direction/purpose (as)
seimbang (dengan) in balance (with)
sekantor (dengan) sharing an office (with)
selaras (dengan) in harmony (with)
sependapat (dengan) of the same opinion (as)
seranjang (dengan) sharing a bed (with)
setara (dengan) equal (to), matching, on the same level (as)

The combination of prefix se- and a noun sometimes forms an adjective meaning “all, the whole (base)”
Examples:
– se-Indonesia
all – Indonesia
– sedunia
(whole) world
– (orang) sekampung
the whole village

2. Prefix se- means “one”. It occurs with group numbers to form cardinals:
sepuluh ten seribu one thousand
sebelas eleven sejuta a million
seratus one hundred

3. Prefix se- also form fractions, often in combination with per-:
seperempat one- quarter setengah one half
seperlima one – fifth separuh one half

While seperdua “half” is possible, setengah and separuh are commonly used.

Classifier “se-”
A classifier precedes a count noun. If the noun is singular, se- “one” precedes the classifier.
Only three classifiers are in frequent use:
orang used with humans seorang guru a teacher
ekor used with living creatures seekor ular/ikan/anjing a snake/fish/dog
buah used with inanimate things sebuah kursi/buku/mobil a chair/book/car

Other examples:
batang used with cylindrical objects such as a pipe, a cigarette sebatang pipa/rokok a pipe/a cigarette

bentuk used with round and curved objects such as a ring, a bangle sebentuk cincin/gelang a ring/a bangle

bidang used with flat, spread-out things such as a plot of land sebidang tanah a plot of land

biji used with round or elongated objects such as fruit or seed sebiji manga a mango

butir used with small round objects (biji and butir sometimes overlap) sebutir pasir a grain of sand

bilah used with sharp things such as knives sebilah pisau a knife

carik used with paper secarik kertas a piece of paper
helai used with flat things like leaves, textiles, paper (helai and carik sometimes overlap) sehelai daun/ kertas a leaf/ a sheet of paper

lembar used with flat things (lembar, helai, carik sometimes overlap) selembar foto a photograph

kuntum used with flowers sekuntum bunga a flower
patah used with words
sepatah dua patah kata a word or two/ one or two words
pucuk used with guns and letters sepucuk senjata/surat a weapon/ a letter
tangkai used with slender objects such as stems setangkai bunga mawar a rose
utas used with rope and thread seutas tali a piece of rope