With other verbs (think, believe), just leave out 'that'. I think this garden is beautiful."
With some verbs (consider) - "I consider this garden to be beautiful."
The relevant point is that you can leave out 'that' in all of them. This is a feature of English that you should look up in a grammar book or webpage, since the answer is not simple and the work's already been done often enough. You could start here: (or search for "omitting that".)
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