[Edit: in 5, I changed “jackdaw” to “magpie”, ale není to
podstatné.]
1. Susan was looking for Graham, so she didn't sit down. Instead, she tried calling / was trying to
call him on her mobile phone. Both options are possible, and
it's not possible to say (solely on the basis of the form of the verb) whether
the trying happened only once or several times. In the absence of an adverbial
marker (such as “repeatedly”, “several times”) we would probably
understand “tried” as “once only”. With “was trying”
it's impossible to say how often the trying occurred. In context, there might
be other clues which would make it clear how often the trying happened.
2. By the time Sheila got back, Chris had gone. / By the time Sheila got
back, Chris was gone. – Je zde stejný význam? Ano. Gone
= pryč.
3. Past perfect is “absolutely necessary” here.
It's the direct reason that the police ignored her complaint. Cf: I couldn't
open my door because I lost / had lost my keys. The present action (ignoring, not being
able to open the door) is the direct consequence of the past
action (phoning many times, losing my keys).
4. Helen felt / was
feeling very tired, and when she finished
her work, she fell asleep. – Tady je v klíči
FELT i WAS FEELING správně. Je to proto, že pocity pomocí FEEL můžeme
vyjádřit jak v prostém tak průběhovém čase a význam se
nezmění? Ano.
5. Nick (lie) down on the grass for a while, next to some tourists who
(feed) the ducks. – Tady má být podle klíče LAY, ale mně tam sedí
i WAS LYING … “Was lying” je přípustné, ale raději bychom řekli
“lay (down)”. “For a while” suggests a completed action in the past (we
know when it ended – after “a while”), and that (not the
simultaneous action of feeding the ducks) governs the tense of “lie down”
here. Nick was lying / lay on the grass for a while watching tourists feed(ing)
the ducks when a magpie [straka] swooped down and flew off with
his phone.
6. Although it rained a lot, I was enjoying /
enjoyed my holiday last year. Again, in the
sentence as it stands, the answer can only be “enjoyed”
because “last year” tells us it's a completed event in the past.
Compare: Although it rained a lot, I was
enjoying my holiday last year until someone stole my umbrella.
It's often difficult (and sometimes impossible) to be precise about the use
of tenses in single sentences taken out of their wider context.