What you need to get:

Best Beginners Dictionary:  Linguee

As well as having great functionality, this app just looks lovely, too.

It's free, and it presents information in a very clear way, for example, look up the English word 'lead'.  You'll see different nouns, verbs, adjectives, all set out beautifully, with grammar detail and contextual examples.

You can tap words to do instant 'reverse-lookups'.

Any drawbacks?  You don't get stress or detail of endings, so checkout the next page for a different app that you can use to help...

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For advanced students:  ReversoContext

This is the (free) app I wish I'd had as a student...   (when you want to know what a native Russian would say, but there's not one around to ask).
Like many dictionary apps nowadays, it uses powerful A.I. algorithms to find translations in context, but it presents them in a very user friendly way. 
Two real game changers for me (that set this apart from the competition): 
1)   it can recognise and search for words in their inflected forms
2)   you can search for whole phrases or sentence fragments, so no more guessing about whether a word can be used in a certain context or structure!
There's a free version, with ads.  But I don't think a serious student will ever regret upgrading to the full version.  Worth its weight in gold.

Read more about what features ReversoContext offers on their website here.