Although the quantity was small, we had……………….to finish the experiment

Answer:

The sentence above is an example of a complex sentence in which shows the relation of contrast. It can be seen from the use of 'although' as a connector. Since the context is in the past time, you need to use the past tense, either past simple or past perfect. This is rather exceptional case as the only possible answer is 'had', which indicates the obligation to finish the experiment. So, the answer would be:
Although the quantity was small, we had had to finish the experiment.

We use 'had' twice because each of them has its own function. The first 'had' is the auxiliary verb of past perfect (had + V3), while the second 'had' is the verb (V3 of 'have to' becomes 'had to'). Such case sometimes do happen, especially if you use that context of obligation in present perfect, past perfect (as above) or future. Here are other example in the other two tenses.
– (present perfect)
One of our friend has been hospitalised for a week, so I've had to cover his classes during his absence.
– (future)
The stadium renovation will have to be done two months before the tournament starts.

Kelas: SMP
Mapel: English
Kategori: past perfect tense, complex sentences