Venus and Earth are very similar in size, bulk density, location relative to our sun (parent star), and that makes us "planetary twins" in one sense. Such similarities are important because Venus did not evolve to be Earth-like for reasons that remain a key question in planetary science. It is possible that Venus was more like Earth in the past, with habitable surface temperatures and oceans of water perhaps up to 300m (984ft) deep (Earth's mean ocean depths are 2700m/8850ft), but over time, the histories of these planets were driven apart.
There are key differences between modern Earth and Venus. First, Venus receives about twice the energy from the sun as Earth so it will naturally be hotter. Its present atmosphere is dominantly CO2 and 90 times thicker than Earth's atmosphere, rather than Earth's unique N2-O2 atmosphere modified by our planet's biosphere, which is an important difference that reflects its possible history post-formation.
Venus has a very thick and globally-encircling, cloud deck with the mid-level clouds being made of sulfuric acid droplets (aerosols) and other not yet well-measured gases. Venus is also a slow rotator relative to Earth and in effect, its solar day is longer than the time it takes (one Venus year) to go around the sun, and it even rotates backward relative to the other planets.  This makes it a very unique planet to compare to our Earth with its 24-hour day and 365.25-day year.
Thus, Venus may be an ideal example of a terrestrial-sized planet that lost its "habitability" over time for various reasons which three upcoming missions will address, each in different ways, in the early to mid-2030s.