How to express singular and plural in Vietnamese (including các vs. những)

1. Indicating singular and plural in general

A noun by itself usually indicates plural

Tôi thích chó (I like dogs)

To express the singular, a classifier like cái, con or chiếc should be added (and optionally, một)

Tôi thấy [một] con chó (I see a dog)

To make a general plural, add one of mấy, [một] vài or một số

Tôi thấy mấy con chó (I see some dogs)

2. Những

The word những is used to indicate a subset of some group, especially in contrast to another subset of that group. If this word is used, the noun always has to be made more specific by some adjective or subclause.

❌ Có những chó ở nhà hàng xóm
✔️ Có một vài/mấy chó ở nhà hàng xóm (There are some dogs in the neighbor's house)
✔️ Có những chó nguy hiểm ở nhà hàng xóm, nhưng chó của tôi không như vậy (There are some dangerous dogs in the neighbor's house, but my dog isn't like that)

In addition to that, những can also be used in combination with question words like gì, ai or đâu as well as with tất cả.

Bạn thường làm những gì trong thời gian rảnh? (What [things] do you usually do in your free time?)
Tôi thích tất cả những chó dễ thương (I like all cute dogs)

3. Các

Các is a type of plural that indicates the entire group, not just a subset. It is usually used when this group is a well-defined and understood to be common knowledge. Typical examples are:

Các ngày trong tuần (Weekdays)
Các màu sắc (Colors)
Các thành viên của ASEAN (The members of ASEAN)

However, there is a second use case with certain nouns like bài viết or nụ hôn. With this type of noun, the classifier is what makes the noun. That is, viết just by itself is the verb "to write", not "articles". So, if you want to turn it into a general, unspecific plural, you cannot just leave out the classifier bài like with con chó. That's when you can use các. It can also be used for foreign words if one doesn't want to or can't translate them into Vietnamese. Examples:

Tôi viết xong bài viết rồi (I finished writing the article)
Tôi viết xong các bài viết rồi (I finished writing the articles)
Tôi đang đọc các blog (I'm reading some blogs)