While some investors are getting excited about the prospects of a new bull market emerging soon, it's good to remember there's no guarantee a rising tide will lift all boats. Sometimes the market's tailwinds may still not ease a company's challenges, while other companies might see their shares take off regardless of whatever is happening with the market. 

Let's take a look at one high-yield dividend stock that probably won't see its prospects improve if a new bull market arrives, and one that might be worth buying before the market booms. 

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Avoid: Medical Properties Trust

With a forward dividend yield of 12.6%, Medical Properties Trust (MPW -1.49%) is an extremely tempting stock that investors should not buy, even if a new bull market comes. The company is a hospital real estate investment trust (REIT) that makes money by buying clinical properties and then leasing them to healthcare businesses. The main idea is that the returns it makes from rents should be reliable enough to provide a good payout to shareholders with the money left over after its expenses.

For that process to be sustainable, its tenants need to pay up on time, and it also needs to be able to consistently scrape together enough cash to buy or build new real estate to build its earnings. Over the last five years, its quarterly funds from operations (FFO) fell by 17%, reaching $124.1 million, so something fundamental isn't working.

One way of generating cash for expansion is by taking out debt. The company currently has $10.4 billion in long-term obligations, much of which will be due in the next few years. But for MPT to have a chance of repaying any debt it takes out, the interest rate it borrows at must be lower than the rate of return it gets from leasing out the new space it purchases. Otherwise, the investment will lose money in the long term. And that's precisely the problem facing this company. 

As interest rates have risen due to the Federal Reserve's attempts to contain inflation, the bar for the necessary returns from each new loan have grown higher and higher. The money left over for returning to shareholders will be harder to come by. At the same time, the prospect of refinancing its existing debt with high interest rates is becoming dimmer. 

That means Medical Properties Trust will need to sell off its holdings to pay off its debt load, like it has several times over the last few quarters. It could also eventually slash its dividend, or perhaps stop increasing it. These issues aren't new, either. Over the last three years, its payout only rose by 7.4%, so shareholders are already experiencing slow growth. So don't buy this stock. 

Buy: NextEra Energy Partners

NextEra Energy Partners (NEP 1.53%) operates renewable and other energy sources, and it's worth buying before the next bull market because the green energy transition is only going to buoy its fortunes. Thanks to subsidies and increased demand for renewables spurred by the recent Inflation Reduction Act (IRA), NextEra's management is estimating an even faster growth rate than before.

And since its dividend yield is above 5.1%, those who buy it now will get a decent return immediately, before even taking future hikes to the payout into account.

The key change that should enable NEP to continue growing its dividend is its decision to transition to only providing renewable energy resources. It'll soon sell off its natural gas pipelines while ramping up onboarding of renewables.

Management expects to continue to be able to grow the dividend at a rate of around 12% to 15% per year through 2026 while maintaining a payout ratio near 80%. That means its earnings could contract by quite a bit, and it would still have the overhead to increase the amount of cash it hands back to investors. That is a good position for a company to be in.

In the last 10 years, its quarterly revenue climbed by 773%, topping $301 million, and more is on the way. As NEP adds to its renewable energy capacity and its inflows grow, its share price may rise too, especially in a bull market. The result will be that its meaty dividend yield falls, so there's an incentive to invest now rather than later.

But in the long term, there will still be a lot of demand for renewable energy, so don't feel rushed into making a purchase. The longer you're willing to hold onto this stock, the more you'll get a return that compounds over time.