the only time you can put the action on the same line as the If...Then
section, is when the entire statement is one line, as

   If something = True Then do this

and you're done.
when you have more than one action for a situation, or one or more Else
sections, you must use the multi-line syntax, as

   If title = "Mr" Then
       gend = "M"
   Else If title = "Mrs" Then
       gend = "F"
   End If

you might want to consider using the Select Case statement, instead, as

   Select Case title
       Case "Mr"
           gend = "M"
       Case "Mrs", "Ms"
           gend = "F"
       Case Else
           gend = "Unknown"
   End Select

btw, what will you do when the title is "Dr"? "Prof"? "Rev"?

hth

There are two kinds of If-Then usages.  One line and a block.  You are mixing
those together.

One line looks like...

If title = "Mr" Then gend = "M"

...and there is NO Else or ElseIf

The block statement looks like...

If title = "Mr" Then
   gend = "M"
ElseIf title = "Mrs" Then
   gend = "F"
End If

Notice that in the block statement you have nothing after the "Then" on the same
line.

There are 2 ways you can write an If statement.

Example1: Entire If ... Then ... on a single line:
   If Amount <= 1 Then Result = "Small amount"

Example 2: Using an If ... Then *block* of code:
   If Amount <= 1 Then
       Result = "Small amount"
   End If

The same applies if the Else is included, e.g.:
   If Amount <= 1 Then Result = "Small amount" Else Result = "Not so small"
or as a If block:
   If Amount <= 1 Then
       Result = "Small amount"
   Else
       Result = "Not so small"
   End If

In practice, I sugges you always use the block, never the single line. The
block is much more powerful (can include lots of lines), and a consistent
style is much easier to debug (i.e. there will always be a matching End If
line for every If statement.)

The colon in VBA does permit mulitple logical lines on one physical line.
For example, you could code:
   A=4: B=3: C=A+B: Debug.Print C
all on the one line. Again, this is poor coding style, and harder to debug.

In your example, Access added the colon between the Else and If because the
previous If line was not a block If. Therefore a new line starting with
ElseIf made no sense to VBA, since a line starting with ElseIf only makes
sense inside a block If.

For your particular case, you might find it easier to use a Select Case
instead of an If block, since it more easily handles a wider range of
values:

   Select Case Me.Title
   Case "Mr", "Master", "Mr."
       Me.gend = "M"
   Case "Miss", "Ms", "Ms.", "Mrs", "Mrs."
       Me.gend = "F"
   End Select

It should look like this, instead:

----------
Public Sub test_ifthenelse()

   Dim title, gend As String

   title = "Mrs"

   If title = "Mr" Then

       gend = "M"

   ElseIf title = "Mrs" Then

       gend = "F"

   End If

End Sub
----------

Notice, I placed space between the lines, and also indented where necessary.
Code indentation is very important, because it's much easier to spot where a
block begins/ends, and especially when you begin nesting loops inside of
Select Case staments, nested in If statements that are nested in other loops
that are nested in If statements, that are also nested inside Select Case
Statements.