When it's is short for it is, it's spelled with an apostrophe. Follow the link to find out more.

solve by reasoning

For more meanings of the phrasal verb work out, follow the link.

Came is the past tense of the irregular verb come. Follow the link to listen to some examples.

Here found is the past tense of the irregular verb find. Follow the link to listen to some examples.

the crawl through the caves is very difficult

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'.

accumulate; gradually increase

For more meanings of the phrasal verb build up, follow the link.

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.