Correct

冬 vs 冬天

/ dōng vs dōngtiān /
Strokes

Definition

冬 and 冬天 both Nouns, which means “winter”.

Example

冬 is an agglutinative word, which cannot be used independently. 冬天 is a free word and can be used freely.
一冬, 冬夜 in examples ① and ② exemplify that 冬 is usually used with other words, such as 暖冬 (warm winter), 冬去春来 (spring follows winter), etc.

冬天 in examples ③ and ④ are used independently.

Except for example ①, the two words are not interchangeable in the other examples.

The difference between 春 and 春天, 夏 and 夏天, 秋 and 秋天 is similar.
1
Jīnnián
今年
dōng
méi
xiàxuě
下雪
  

There is no snow through all the winter.

2
Zài
zhège
这个
jìng
寂静
de
dōng
冬夜
  
,
jiārén
一家人
wéi
围炉
pǐnchá
品茶
  

On the still winter night,the family sat around the oven over tea.

3
Dōngtiān
冬天
dào
liǎo
  

Winter is coming.

4
Zhè
shì
hánlěng
寒冷
de
dōngtiān
冬天
  

This is a harsh winter.