Written addition and subtraction to 1000

Line up the columns, not the edges

When you write $246+38$ vertically, the $8$ goes under the $6$ — units under units. Lining numbers up on the left is the single most common cause of wrong answers at this stage.

Carrying

$246+38$: units give $6+8=14$. Fourteen is one ten and four, so write $4$ and carry the $1$ into the tens column. Carrying is not a trick — it is just exchanging ten units for one ten.

Exchanging when subtracting

$305-128$: you cannot take $8$ from $5$, so exchange one ten for ten units. The tens column here is $0$, so you first exchange one hundred for ten tens. Doing it in two clear steps beats trying to do it in one.

Estimate first

$246+38$ is roughly $250+40=290$. If your written answer is $626$, the estimate catches it instantly.