Tenths are pieces
0.3 is three tenths: cut a bar into 10 equal parts and color 3 of them. 0.4 is four tenths of the same bar.
Two bars, each cut into 10 equal parts: 3 tenths are colored on the first and 4 tenths on the second.
Add the pieces
To work out 0.3 + 0.4, put the colored pieces together and count them: 3 + 4 = 7. The pieces are still tenths, so the answer is 7 tenths, which is written 0.7.
It works like adding any matching things. 3 apples and 4 apples are 7 apples, and 3 tenths and 4 tenths are 7 tenths. The kind of piece does not change when you add.
The 3 tenths and the 4 tenths together color 7 of the 10 parts: 0.7.
Line up the points
Written one above the other, the decimal points line up. Then the tenths digits sit in the same column and are added to each other: 3 + 4 = 7 in the tenths column, and 0 + 0 = 0 in the ones. The point comes straight down into the answer: 0.7.
The points are lined up, so the 3 tenths and the 4 tenths are in one column. They make 7 tenths.
Two slips
Leaving out the point gives 7. That is 7 whole bars, but only 7 tenths of one bar are colored.
Writing 0.07 turns the answer into hundredths, pieces ten times smaller. Adding tenths to tenths gives tenths, so the answer is 0.7.