Somebody handed me this (working!) solution I like to share:

with(plots):
p1:=animate(arrow,[<cos(tend),sin(tend),0.9*tend>,<-cos(tend),-sin(tend),0>,color=red],tend=0..50,axes=boxed,frames=150);
alpha:=1:
p2:=animate(textplot3d,[[[-1.8,0,alpha*tend,piecewise(tend<=12,"",tend<=25,"position","")]]],tend=0..50,axes=boxed,frames=150);
display(p1,p2);

Thanks!

Thanks for answers. Allow a specified (clearer?) question. I have an animated helix/ arrow:

p1:=animate(arrow,[<cos(tend),sin(tend),0.9 tend>,<-cos(tend),-sin(tend),0>,color=red],tend=0..50,axes=boxed,frames=150);

With some (parallel running) textlines I want to clarify a few phases: what happens when:

while tend <= 12; 12<=tend<=25;  25=>tend<=50. something like: while tend <= 12 do text1 or while 12<=tend<=25 do text2.

of course I can do this:

pos:=animate(textplot3d,[[-1.8,0,alpha*tend, 'position',z=12..25]], tend=12..25,frames=150);

But text 'position' always displays. I want the text only to appear between tend =12 and tend = 25.

I have this vision of explaining in graph, but no skills in (complex?) loops. So, again, please help?

Thanks for answers. Allow a specified (clearer?) question. I have an animated helix/ arrow:

p1:=animate(arrow,[<cos(tend),sin(tend),0.9 tend>,<-cos(tend),-sin(tend),0>,color=red],tend=0..50,axes=boxed,frames=150);

With some (parallel running) textlines I want to clarify a few phases: what happens when:

while tend <= 12; 12<=tend<=25;  25=>tend<=50. something like: while tend <= 12 do text1 or while 12<=tend<=25 do text2.

of course I can do this:

pos:=animate(textplot3d,[[-1.8,0,alpha*tend, 'position',z=12..25]], tend=12..25,frames=150);

But text 'position' always displays. I want the text only to appear between tend =12 and tend = 25.

I have this vision of explaining in graph, but no skills in (complex?) loops. So, again, please help?

Thank you so much! works like a charm.  So for everybody interested:

restart:with(plots):

p4:=animate(arrow,[<cos(tend),sin(tend),0.9 tend>,<-cos(tend),-sin(tend),0>,color=red],tend=0..50,axes=boxed,frames=150);

l3:=animate(textplot3d,[[cos(tend),sin(tend), 0.9 tend, 'basis'],color=black,align={BELOW},font=[TIMES,ROMAN,14]],tend=0..50,frames=150);

l4:=animate(textplot3d, [[0, 0, 0.9*tend,'direction'], color=red, align={BELOW}, font=[TIMES,ROMAN,14]], tend=0..50, frames=150):

display([p4,l3,l4]);

Thank you so much! works like a charm.  So for everybody interested:

restart:with(plots):

p4:=animate(arrow,[<cos(tend),sin(tend),0.9 tend>,<-cos(tend),-sin(tend),0>,color=red],tend=0..50,axes=boxed,frames=150);

l3:=animate(textplot3d,[[cos(tend),sin(tend), 0.9 tend, 'basis'],color=black,align={BELOW},font=[TIMES,ROMAN,14]],tend=0..50,frames=150);

l4:=animate(textplot3d, [[0, 0, 0.9*tend,'direction'], color=red, align={BELOW}, font=[TIMES,ROMAN,14]], tend=0..50, frames=150):

display([p4,l3,l4]);