con

When counting most physical objects in Vietnamese, you need a classifier (though some nouns, like time units such as ngày or tháng, do not use one). The standard counting pattern is:

Number + Classifier + Noun

Classifiers are also often used when using demonstratives (like này, đó, kia), even without numbers, though some nouns can attach directly to demonstratives:

Classifier + Noun + Demonstrative

Con is one of the most common classifiers. Here is how it is used:

• Animals: It is the default classifier for animals, birds, fish, and insects (e.g., hai con mèo = two cats). However, animals are sometimes personified using human kinship terms instead (e.g., chú chó = dog, bác gấu = bear).
• Spirits and Monsters: It is used for supernatural beings (e.g., con ma = ghost, con quái vật = monster).
• People: It appears in general terms for people (con người = human). It is also used informally to refer to young girls (e.g., con bé đó = that girl), while the male equivalent is thằng (e.g., thằng bé đó = that boy). However, using con as a classifier for an adult woman is highly derogatory and disrespectful when used for a stranger, colleague, or someone of equal or higher status. It is, however, perfectly normal and neutral or affectionate when used by older relatives or very close peers. Note that the standard, neutral classifier for counting people by their profession or status is người (e.g., hai người giáo viên = two teachers). Using con to count professionals is highly offensive. Additionally, it is used for gender-neutral negative roles (e.g., con nợ = debtor, con bạc = gambler).
• Specific Objects: Con is not just for living things! It is used for objects implying movement, action, or an active role (con sông = river, con đường = road, con dao = knife, con thuyền = boat, con diều = kite). Note that while chiếc is the standard classifier for most vehicles, con is traditionally used for boats. It is also used for a few specific body parts as exceptions (con mắt = eye, con tim = heart), though the vast majority of body parts use the default inanimate classifier cái. Finally, there are lexicalized exceptions that simply need to be memorized, such as con tem (stamp), con cờ (chess piece), con số (number), con sóng (wave), and con chữ (written letter/character). Note that con chuột (computer mouse) takes con because it is a direct metaphorical extension of the animal rule (named after a mouse).
• Modern Slang: Young Vietnamese frequently use con instead of cái or chiếc as a highly common, casual slang marker for technology, gadgets, prized possessions, and vehicles (e.g., con điện thoại = phone, con laptop = laptop, con xe = ride/vehicle).

Finally, note that con also functions as a noun root meaning 'child' or 'offspring' (as seen in con trai = boy/son, con gái = girl/daughter, con nít = children), and as a standalone kinship pronoun used to mean 'I' when speaking to parents or elders, or 'you' when parents or elders are speaking to a child or younger person.