It's
When it's is short for it is, it's spelled with an apostrophe. Follow the link to find out more.
more information: its & it's
work out
solve by reasoning
Example: I can't work out what went wrong.
For more meanings of the phrasal verb work out, follow the link.
more information: work out
came
Came is the past tense of the irregular verb come. Follow the link to listen to some examples.
more information: come
found
Here found is the past tense of the irregular verb find. Follow the link to listen to some examples.
more information: find
the going's not easy
the crawl through the caves is very difficult
were falling
We use was or were with the ‘ing’ form of a verb to talk about something that happened in the past and continued for a while. The animals kept falling down into the caves over a long period of time. To talk about them falling in the past once, you say that 'they fell' into the cave, but to talk about a number of such instances over time you say that they 'were falling'.
more information: past continuous
build up
accumulate; gradually increase
Example: Silt builds up in the dam.
For more meanings of the phrasal verb build up, follow the link.
more information: build up
its
Notice that its is not spelled with an apostrophe here because it is not a contraction of it is. Follow the link to find out more.
more information: its & it's