[Carlo Lazzaro]

Joachim:
try:

xtreg x1 x2 x3 x4 x5 x6 i.year

It's still unclear why you want to have both linear time trend and time fixed effect in your code.

Your second code, as expected, gives you troubles because it is far too complicated (and basically meaningless).
I'm still under the impression that you're trying to pushing codes to the limit without a clear research goal.

[Joachim Vanderheyden]

Carlo:
I want to test that to see if there is a difference with the code with linear time trend and without year fe

Than do I need to change that code.

If this code " xtreg x1 x2 x3 x4 x5 x6 i.year " is with linear time trend, with year FE and without corporate FE is. If I understand everything than is the next code:
xtreg x1 x2 x3 x4 x5 x6 i.year, fe

Time trend: Yes
Corporate FE: Yes
Year FE: Yes

Is that correct?

[Carlo Lazzaro]

Joachim:
1) code with linear time trend, with year FE and without corporate FE:

xtreg x1 x2 x3 x4 x5 x6 c.year i.year

2) code without linear time trend, with year FE and without corporate FE:

xtreg x1 x2 x3 x4 x5 x6 i.year

3) code with linear time trend, without year FE and without corporate FE:

xtreg x1 x2 x3 x4 x5 x6 c.year

[Joachim Vanderheyden]

Carlo:
Now it is completely clear to me. Thank you very much for your help.