Putting Amounts in Month Columns

Chris Constantinou's excellent formula for cell C2 was
    =IF(MONTH($A2)=COLUMN(A$1),$B2,0)
Here is a simpler version
    =(MONTH($A2)=COLUMN(A$1))*$B2
This works because Excel converts TRUE/FALSE to 1/0 when doing arithmetic. (Note the numeric value of True in VBA is -1, not 1.)
My recent comment below described this array formula inspired by Chris
    =IF(MONTH($A2)=COLUMN($A:$L),$B2,0)
Here is the simpler version of this array formula
    =(MONTH($A2)=COLUMN($A:$L))*$B2
In newer versions of Excel this formula will automatically spill from cell C2 as a dynamic array in cells C2:N2. The array formula in cell C2 can be copied to fill additional rows below row 2.
In older Excel versions it is necessary to select cells C2:N2 before entering the formula with Ctrl+Shift+Enter, which makes it a CSE array. The CSE array formula in C2:N2 can then be copied to fill additional rows.

@Steve Jez
I appreciate your comment, but many of us consider discussions like this as "fun with Excel" because there are so mnay different ways to achieve a result.

I appreciate the original post asked for a formulaic approach, but, without wishing to sound harsh or rude, I do wonder if sometimes if a general lack excel knowledge or understanding has prompted the question. In that, this is the way I currently do it manually & I'd like to replace it with a formula. And it's great that people pile in with answers & then improvements on those answers as already shown in this thread.
However, on the brief information we're given here, I can think of no logical reason to split the transactions into monthly columns other than to be able to sum the months for monthly totals.

Based on this example I would enter the initial data - Date Amount in a table, with one additional column for the month calculation. Then use a summarising formula to provide a month by month running total. This way you end up with far fewer formulas to calculate every time a new entry is made.

Summarising formulas in case the image is not clear, the first 2 are required regardless of which of the summarising formulas are used - Filter - Sumifs - Sum

Column A =MONTH([@Date])

Column E =TEXT(E3*29,"MMM")

Column G =SUM(FILTER(Table1[Amount],MONTH(Table1[Date])=E3,0))
Column H =COUNT(FILTER(Table1[Amount],MONTH(Table1[Date])=E3,""))

Column J =SUMIFS(Table1[Amount],Table1[Month],E3)
Column K =COUNTIFS(Table1[Month],E3)

Column M =SUM((MONTH(Table1[Date])=E3)*Table1[Amount])

If months & years were the criteria, it would be a pretty simple amendment.

I hope I haven't missed the point & this post is only intended to help.
(see Figure 1 below)

For text months in the heading rather than relying on column position you could do something like this:
• =IF(TEXT($A2,"mmm")=C$1,$B2,0) <--- for Jan, Feb etc
•=IF(TEXT($A2,"mmmm")=C$1,$B2,0) <--- for January, February etc
or still working with text months if you have MS365 using the spill function with the formula in C2
• =IF(MONTH($A$2:$A$16)=MONTH("01-"&$C$1:$N$1),$B$2:$B$16,0)
•=IF(TEXT($A$2:$A$16,"mmm")=$C$1:$N$1,$B$2:$B$16,0) <--- for Jan, Feb etc

Here is another approach, without any formulas in the sheet, by using this tiny event macro:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column <> 2 Then Exit Sub
If IsDate(Target.Offset(0, -1)) = False Then Exit Sub
Cells(Target.Row, Month(Target.Offset(0, -1)) + 2) = Target
End Sub

To use it, you must first enter the date then the amount. This amount is then automatically copied in the right month column.

If you want to change a row, first delete the amount. The corresponding month amount will also be deleted. Then you can enter a new date and corresponding amount.

The header can be a traditional one with any words of your choice.

You can use any row in the column range A:N without any need to copy formulas.

The columns not used in a row stay empty so the file will be much smaller in size.

Remember to put his macro in the sheet’s code page. Right click the sheet tab an select “View Code”. That’s the place to be.

My initial comment below discusses the following array formula
    =IF(MONTH($A2)=(COLUMN($C:$N)-2),$B2,0)
As suggested by Chris Constantinou, here is a simpler version
    =IF(MONTH($A2)=COLUMN($A:$L),$B2,0)

My contribution to this tip was the following formula in C2
=IF(MONTH($A2)=COLUMN(A$1),$B2,0)
which eliminates the need to subtract 2 from the COLUMN function result.

The Tip's last formula intended for cell C2 is
    =IF(MONTH($A2)=(COLUMN(C$1)-2),$B2,0)
This can be simplified as follows
    =IF(MONTH($A2)=(COLUMN()-2),$B2,0)
The advantage of both is their independence of the contents of header row 1. After entering either formula in cell C2, it can be copied to columns C:N in rows below row 1.
Here is an equivalent array formula for cell C2
    =IF(MONTH($A2)=(COLUMN($C:$N)-2),$B2,0)
In newer versions of Excel this formula will automatically spill from cell C2 as a dynamic array in cells C2:N2. The array formula in cell C2 can be copied to fill additional rows below row 2.
In older Excel versions it is necessary to select cells C2:N2 before entering the formula with Ctrl+Shift+Enter, which makes it a CSE array. The CSE array formula in C2:N2 can then be copied to fill additional rows.